Remove build.number.maven

I can't find any justification for having this information
in both build.number and build.number.maven.

They have drifted apart on the 2.10.x branch, although that
doesn't matter because build.number is correct, and loaded first,
and Ant properties are write-once.

I'm assuming that the Ant tasks in src/build/pack.xml are only
invoked through the <antcall>-s in ./build.xml.