cmake: stop rebuilding rocksdb everytime

cmake: stop rebuilding rocksdb everytime

this change was originally introduced as a part of
418bfd7bb5ec1dcec2b011e9df118c33ce38d398, and latter migrated / changed
in the current form. but the idea is the same: to rebuild rocksdb even
if the stamp file shows that it has been built. there is no need to do
so, as we don't hack RocksDB as we used to. also,it is distracting to
check this log message when rebuilding the tree. so drop it.

Signed-off-by: Kefu Chai <kchai@redhat.com>