Removing AbuseFilter::checkConditions'  param

I'm not really sure why it exists; I assume to minimize overhead of init'ing
multiple AbuseFilterParser objects that would be exactly the same.

However, checkConditions() - which checks one specific condition - is not really
an "endpoint" you will want to call in <your-extension-that-uses-AbuseFilter>.
You're more likely to call something higher-lever (like
AbuseFilter::filterAction, which will take care of fetching all appropriate
filters, as well as run them through checkConditions, and much more). This will
trickle down to ::checkAllFilter, down to ::checkFilter, which would eventually
call ::checkConditions with the 'keepvars' argument.
Basically, unless you're re-implementing much of AbuseFilter yourself, you won't
get to pass anything other than 'keepvars' to checkFilter.
As a result, even though you may call AbuseFilter to call on multiple different
vars, it will re-use the same parser with the first vars.

I'm proposing to drop the 'keepvars' and just keep the vars around instead.
checkFilter can compare previous vars with new vars, and only init a new Parser
if the vars are different.

Change-Id: I96ccc60c77f3cdbb82c0f9f16782a1a44ffb1592
(cherry picked from commit 84536df324ecbd04a1f1150901c0c26b219039d2)