I (tried to) keep the RTF filter style consistent locally with astyle in
the past, but now that's redundant when we have an enforcing
clang-format mechanism in place. So drop the astyle config and switch to
clang-format in the RTF filter case.

To minimize backport pain, do this shortly before the libreoffice-6-0
branch-off.

Change-Id: I708dbeb0b5ad2afacc90029ee5abba9495f4601f