encoding: delete v1 proto codec and use one map for registry

[dfawley]

cc @PapaCharlie

Without this change, existing users that overwrote the v1 "proto" codec with special changes will have a behavior change, as we will choose the v2 "proto" codec over theirs. Also there seems to be little value to keeping the v1 proto codec around since we would never use it.

RELEASE NOTES: none

[codecov]

Codecov Report

Attention: Patch coverage is 75.86207% with 7 lines in your changes missing coverage. Please review.

Project coverage is 81.71%. Comparing base (3d95421) to head (0ede3f6).
Report is 1 commits behind head on master.

Files	Patch %	Lines
encoding/proto/proto.go	72.72%	4 Missing and 2 partials ⚠️
codec.go	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7557      +/-   ##
==========================================
- Coverage   81.79%   81.71%   -0.09%     
==========================================
  Files         362      361       -1     
  Lines       27832    27816      -16     
==========================================
- Hits        22765    22729      -36     
- Misses       3856     3875      +19     
- Partials     1211     1212       +1     

Files	Coverage Δ
encoding/encoding.go	56.25% <100.00%> (-17.09%)	⬇️
encoding/encoding_v2.go	60.00% <100.00%> (+4.44%)	⬆️
codec.go	10.52% <50.00%> (-72.09%)	⬇️
encoding/proto/proto.go	66.66% <72.72%> (ø)

... and 19 files with indirect coverage changes