Go: Rename `Size_` back to `Size`

[ashleyvega]

The generated Golang code does not work.

See also mwitkow/go-proto-validators#51

Fix

Fix it with something like:

find proto -name '*.validator.pb.go' | sort | while read -r pbfile
do
	sed -i -re 's/this\.Size_/this.Size/' "$pbfile"
done