Fix data race in genny list Remove

[ryanhall07]

The data race is reading the value after the element has been put back
in the pool. Another goroutine can get the element out of pool before
the value is read again.

This was not causing an actual production issue because no caller
actually uses the returned value from Remove. However, it did cause
data race failures in tests when run with -race.

WARNING: DATA RACE
Write at 0x00c063a621d8 by goroutine 3582:
  github.com/m3db/m3/src/x/context.(*finalizeableList).insertValue()
      /Users/rhall/go/src/github.com/m3db/m3/src/x/context/finalizeable_list_gen.go:202 +0xa8
  github.com/m3db/m3/src/x/context.(*finalizeableList).PushBack()
      /Users/rhall/go/src/github.com/m3db/m3/src/x/context/finalizeable_list_gen.go:239 +0x96
  github.com/m3db/m3/src/x/context.(*ctx).registerFinalizeable()
      /Users/rhall/go/src/github.com/m3db/m3/src/x/context/context.go:135 +0xce
  github.com/m3db/m3/src/x/context.(*ctx).RegisterFinalizer()
      /Users/rhall/go/src/github.com/m3db/m3/src/x/context/context.go:109 +0xa2
  github.com/m3db/m3/src/dbnode/storage/index.(*block).aggregateWithSpan()
      /Users/rhall/go/src/github.com/m3db/m3/src/dbnode/storage/index/block.go:703 +0x690
  github.com/m3db/m3/src/dbnode/storage/index.(*block).Aggregate()
      /Users/rhall/go/src/github.com/m3db/m3/src/dbnode/storage/index/block.go:607 +0x20a
  github.com/m3db/m3/src/dbnode/storage.(*nsIndex).execBlockAggregateQueryFn()
      /Users/rhall/go/src/github.com/m3db/m3/src/dbnode/storage/index.go:1763 +0x611
  github.com/m3db/m3/src/dbnode/storage.(*nsIndex).execBlockAggregateQueryFn-fm()
      /Users/rhall/go/src/github.com/m3db/m3/src/dbnode/storage/index.go:1736 +0x150
  github.com/m3db/m3/src/dbnode/storage.(*nsIndex).queryWithSpan.func1()
      /Users/rhall/go/src/github.com/m3db/m3/src/dbnode/storage/index.go:1589 +0x22f
  github.com/m3db/m3/src/x/sync.(*workerPool).GoWithContext.func1()
      /Users/rhall/go/src/github.com/m3db/m3/src/x/sync/worker_pool.go:128 +0x34
Previous read at 0x00c063a621d8 by goroutine 2940:
  [failed to restore the stack]
Goroutine 3582 (running) created at:
  github.com/m3db/m3/src/x/sync.(*workerPool).GoWithContext()
      /Users/rhall/go/src/github.com/m3db/m3/src/x/sync/worker_pool.go:127 +0x2e6
  github.com/m3db/m3/src/dbnode/storage.(*nsIndex).queryWithSpan()
      /Users/rhall/go/src/github.com/m3db/m3/src/dbnode/storage/index.go:1587 +0x8f2
  github.com/m3db/m3/src/dbnode/storage.(*nsIndex).query()
      /Users/rhall/go/src/github.com/m3db/m3/src/dbnode/storage/index.go:1467 +0x279

What this PR does / why we need it:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:

Does this PR require updating code package or user-facing documentation?:

[codecov]

Codecov Report

Merging #3233 (4d6f7a1) into master (d4ff5a6) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##           master    #3233   +/-   ##
=======================================
  Coverage    72.3%    72.3%           
=======================================
  Files        1094     1094           
  Lines      101122   101123    +1     
=======================================
+ Hits        73190    73196    +6     
  Misses      22868    22868           
+ Partials     5064     5059    -5     

Flag	Coverage Δ
aggregator	75.8% <ø> (-0.1%)	⬇️
cluster	84.8% <ø> (-0.1%)	⬇️
collector	84.3% <ø> (ø)
dbnode	78.8% <ø> (+<0.1%)	⬆️
m3em	74.4% <ø> (ø)
m3ninx	73.3% <ø> (-0.1%)	⬇️
metrics	20.0% <ø> (ø)
msg	74.2% <ø> (+0.3%)	⬆️
query	67.3% <ø> (ø)
x	80.2% <100.0%> (+<0.1%)	⬆️

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[robskillington]

LGTM, great to have this noise out of the way