Remove OutputPage::uncacheableBecauseRequestvars()

Remove OutputPage::uncacheableBecauseRequestvars()

Fix for r10265: OutputPage::uncacheableBecauseRequestvars() as written
in that revision is backwards, apparently declaring all requests to be
uncacheable except those with useskin or uselang. Luckily getText() has
always converted its default parameter to a string, so the triple-equals
comparisons always fail, and uncacheableBecauseRequestvars() always
returns false.

In any case, it was never necessary to suppress the client-side cache
for useskin/uselang views, so the accidental behaviour was appropriate.

Change-Id: I520940867509b986a47d63ead9b549e8aa82fa1f