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
diff --git a/BUILD b/BUILD
index dc73a1d2..f44d7d7 100644
--- a/BUILD
+++ b/BUILD
@@ -13,7 +13,6 @@
     srcs = glob(["src/main/java/**/*.java"]),
     manifest_entries = [
         "Gerrit-PluginName: " + plugin_name,
-        "Gerrit-ApiVersion: 2.16",
         "Implementation-Title: Task Plugin",
         "Implementation-URL: https://gerrit-review.googlesource.com/#/admin/projects/plugins/" + plugin_name,
         "Gerrit-Module: com.googlesource.gerrit.plugins.task.Modules$Module",