Correct logical associated with NW2 tracers

[adcroft]

• @klindsay28 spotted two issues for the NW2 tracers
  1. Use of the wrong logical variable
  2. Incorrect comment
• Using the wrong logical meant that the ideal age pacakge was being called in addition to the NW2 package, but did not affect the NW2 tracers themselves.

[codecov]

Codecov Report

Merging #1518 (554c8a0) into dev-gfdl-main-candidate-2021-10-04 (de7a771) will not change coverage.
The diff coverage is 100.00%.

@@                         Coverage Diff                         @@
##           dev-gfdl-main-candidate-2021-10-04    #1518   +/-   ##
===================================================================
  Coverage                               29.06%   29.06%           
===================================================================
  Files                                     237      237           
  Lines                                   71643    71643           
===================================================================
  Hits                                    20823    20823           
  Misses                                  50820    50820           

Impacted Files	Coverage Δ
src/tracer/MOM_tracer_flow_control.F90	51.87% <100.00%> (ø)

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[Hallberg-NOAA]

I agree that these changes are correct and should be merged in.