Stop double adding Gerrit-ApiVersion

Ever since Change-Id: I6f17e6c6bbf, bazlets has support for adding the
ApiVersion to the MANIFEST.MF file for us and I0a786a91147 added support
in core for in-tree builds before that.

Because of that support and us also adding it manually, we end up with
two (possibly conflicting) Gerrit-ApiVersion entries in the MANIFEST.MF
file. Fix that and simplify our tracking too by removing our manual
entry from our BUILD file.

Change-Id: If7f1af20a263643ec89179cdd8fe94d20d523366
1 file changed