Removes "install_..._docs" and "clean_docs" special targets.
Instead install documentation with "--install . --component qch_docs"
and "--install . --component html_docs" which have the advantage that
the install prefix can be specified later on with --prefix.
Also removes export of IDE_SOURCE_DIR, which is not used.
Change-Id: I4fce0b9aa8c7fc40b7e94e065777ddf4c0b76c41
Reviewed-by: Cristian Adam <cristian.adam@qt.io>