doc: Clean up Doxyfile for doxygen 1.8.6

doc: Clean up Doxyfile for doxygen 1.8.6

Latest stable is v1.8.8, but Ubunu Trusty (which we now use on the
Jenkins slave that generates our documentation) ships doxygen v1.8.6.
Time to migrate away from 1.7.x stuff.

Warnings:

 Tag `SYMBOL_CACHE_SIZE' at line 84 of file maintenance/Doxyfile has become obsolete.
 Tag `SHOW_DIRECTORIES' at line 118 of file maintenance/Doxyfile has become obsolete.
 Tag `HTML_ALIGN_MEMBERS' at line 226 of file maintenance/Doxyfile has become obsolete.
 Tag `USE_INLINE_TREES' at line 253 of file maintenance/Doxyfile has become obsolete.

Regenerated with:

 $ doxygen -u -s ./maintenance/Doxyfile

Which automatically added new configurations settings, removed
obsolete ones, migrated formatting to their new standard.

Change-Id: I372ea06c439762578cb4c68686344eb446925c04