libs.doxy_template: remove some obsolete variables
libs.doxy_template: remove some obsolete variables

- COLS_IN_ALPHA_INDEX
  this was unused given ALPHABETICAL_INDEX = NO
- PERL_PATH / MSCGEN_PATH
  these were unused

quiets warnings with doxygen 1.9.1:
warning: Tag 'COLS_IN_ALPHA_INDEX' at line 1110 of file 'doxyfile' has
become obsolete.
warning: Tag 'PERL_PATH' at line 1105 of file 'doxyfile' has become
obsolete.
warning: Tag 'MSCGEN_PATH' at line 1126 of file 'doxyfile' has become
obsolete

Change-Id: I6229311afaa3318a3f9bcaf40fafcc5ea71ae271

webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx