removed future_rev_count counter which is undefined and unused
removed future_rev_count counter which is undefined and unused

The example file in `example/lib.persistence.api.py` does not run because the
`mw/lib/persistence/api.py` file includes a line incrementing the variable
future_rev_count which is undefined. Because the variable is not referred to
elsewhere, it seems safe to remove.

makoshark/Mediawiki-Utilities,mediawiki-utilities/python-mediawiki-utilities,halfak/Mediawiki-Utilities