balancergroup: remove mentions of locality from comments

[easwars]

The balancergroup was initially implemented inside the priority LB policy, before being moved out to be a separate utility. Mentions of locality are from that day and age.

RELEASE NOTES: none

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 81.42%. Comparing base (887d908) to head (a623b7a).

Files	Patch %	Lines
internal/balancergroup/balancergroup.go	50.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7476      +/-   ##
==========================================
- Coverage   81.53%   81.42%   -0.12%     
==========================================
  Files         357      357              
  Lines       27242    27242              
==========================================
- Hits        22213    22183      -30     
- Misses       3827     3846      +19     
- Partials     1202     1213      +11     

Files	Coverage Δ
internal/balancergroup/balancergroup.go	79.81% <50.00%> (-1.41%)	⬇️

... and 16 files with indirect coverage changes