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: I8b1829c719f10a08227980468f01703544a7f269
diff --git a/BUILD b/BUILD
index 9acc76b..b224283 100644
--- a/BUILD
+++ b/BUILD
@@ -5,7 +5,6 @@
     srcs = glob(["src/main/java/**/*.java"]),
     manifest_entries = [
         "Gerrit-PluginName: batch",
-        "Gerrit-ApiVersion: 2.16",
         "Implementation-Title: Batch Plugin",
         "Implementation-URL: https://gerrit-review.googlesource.com/#/admin/projects/plugins/batch",
         "Gerrit-Module: com.googlesource.gerrit.plugins.batch.Module",