Remove old ep_version stat

As a result of the merge of the ep-engine & memcached repos, the separate ep_version statistic - the git SHA of ep-engine - is no longer needed.

In fact, it currently reads as unknown:

$ cbstats localhost:11210  -u Administrator -p asdasd -b default all|grep ep_version

 ep_version:                                            unknown

We should therefore just remove this statistic, as it just adds noise.