Don't run the non-Tidy "bug 2702" hack unless Tidy is really missing

Don't run the non-Tidy "bug 2702" hack unless Tidy is really missing

We have two hacks which are used when Tidy is not available: one in
Sanitizer::removeHTMLtags(), and the second here as a late Parser pass
equivalent to Tidy itself. But the Sanitizer one was enabled only if
MWTidy::isEnabled() returned false, whereas the Parser one was enabled
also when tidy was disabled in ParserOptions. This patch makes them both
consistent, it enables the bug 2702 hack only when MWTidy::isEnabled()
returns false, and when Tidy is disabled in parser options, the output
is simply passed through.

This allows tidying to be done separately on the ParserOutput, as is
required by the proposed ParserMigration extension (I24d0776a933fa3f).

Eventually the bug 2702 hack will be removed in favour of a pure-PHP
HTML 5 parser, but it looks like it is too early for that.

Change-Id: I94be6c9dec531c23ef80cb36732243bd6858bf22