Remove BatcacheWhitelistedParamsSniff

[rebeccahum]

Same reason as #612 — no longer needed since WordPress.com sites are migrated.

[jrfnl]

Same as for #612:

Action plan suggestion:

1. Deprecate the sniff in the next release (2.x) by:
   1. Adding a deprecation warning to the sniff which will inform people to remove the sniff reference from their own custom ruleset.
   2. Silencing all messages from the sniff from within the VIPMinimum ruleset, effectively removing the sniff already for people using the standard rulesets.
2. Remove the actual sniff in the next major, i.e. 3.0.0.

Loosely related to #600