docs: Remove unused PERL_PATH and unused file patterns from Doxyfile

docs: Remove unused PERL_PATH and unused file patterns from Doxyfile

* File patterns - Keep only ".php" and ".inc". The other file types
  either don't exist in this repo, or exist but have 0 parseable doc
  blocks for Doxygen to find. Also remove README, it leads to various
  pointless attempts for Doxygen to parse files by this name without
  any documentable entity in them. Such as;

  > mediawiki/includes/jobqueue/README:82: warning: Reached end of file
  > while still inside a nested comment. Nesting level 1.

* Remove 'PERL_PATH'. This has been unused since Doxygen 1.8.0,
  the last build on doc.wikimedia.org from Jenkins uses Doxygen 1.8.13.
  As of Doxygen 1.8.16 a deprecation warning was also added,
  https://github.com/doxygen/doxygen/commit/6d1535c.

Change-Id: I99bd80f9cf7bae8d37caf0b9325bd30a16020497