reviewdb.proto: Remove unused Protobuf messages
reviewdb.proto: Remove unused Protobuf messages

Some of the Protobuf messages aren't used anywhere in our code base.
Hence, remove them.

Change-Id: Idd31a2096f60c506c2ce73b0dbecaa2ec2518bcb

qtproject/qtqa-gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,qtproject/qtqa-gerrit