internals: properly remove all docs

Part of the installed doc was not removed (if configured to).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from cb0f625b3f9f704140aaeed024baac05e6b27df0)

reporter_name="Alexander BIGGA"

reporter_url="http://sourceware.org/ml/crossgcc/2008-06/msg00031.html"

reporter_comment=""