Remove gpaddmirrors from NON_PRODUCTION_FILES.txt

Include gpaddmirrors in the distribution.
Co-authored-by: NKalen Krempely <kkrempely@pivotal.io>
Co-authored-by: NMark Sliva <msliva@pivotal.io>

gpAux/releng/NON_PRODUCTION_FILES.txt

 bin/explain.pl
 bin/explain.pm
-bin/gpaddmirrors
 bin/gptorment.pl
 bin/lib/gptest.py
 bin/pgbench