Remove unused 'VERSIONS' definition

After making template formats pluggable in patch
5709e08f33406cdd40b0b4e6fbd7fd2b8008976e, the 'VERSIONS' is
unused in /hot/template.py, we should remove it.

Change-Id: Icfdc0ccd7159bd521d9fa1f5e62a8a757e12bbe7