doc: remove directives deprecated in Doxygen 1.9.5

All these Doxygen configuration directives raise a deprecation warning with Doxygen 1.9.5.

Since we have WARN_AS_ERROR = YES, this causes the build to fail.

Remove these deprecated directives.

I have checked the differences by first building from scratch without this patch, and then building from scratch with this patch, and in the latter builddir checking

$ diff -ru -x '*.md5' -x '*.pdf' ~/tmp/weston-doc-before doc

The only differences are the Doxygen config file and one .pickle file. So it seems the generated docs are identical with Doxygen 1.9.1.

Fixes: #661 (closed)