Cleanup before protoc/grpc artifacts push (#3207)
Cleanup before protoc/grpc artifacts push (#3207)

(this is to separate cleanup changes from the actual proto/grpc artifacts push, which is huge)
1) Cleanup `pom.xml`s (remove unused/duplicate, partially wrong configuration)
2) Update `versions.txt` to include `proto/grpc` artifacts version (will be pushed immediately after this change is pushed). Also update `bump_versions.py` and `replace_versions.py` to strip whitespaces and support empty lines.
3) Fix License to pint explicitly to `Apache 2.0` license (instead of Google Software License, which contains apache license in it (did compare the text, it is identical). Otherwise all third-party tools do not recognize the license (including github, looks like).
4) Add Andrea and Hanzhen to developers list.
googleapis/google-cloud-java,googleapis/google-cloud-java,googleapis/google-cloud-java