checkdocs: fix the handling of the local kdoctools sources

Remove the hack which used to rely on XDG_DATA_DIRS, which
does not seem to work with kdoctools or most likely the Qt
version in bionic (maybe some QStandardPath change?)
and simply rely on the --srcdir meinproc parameter,
which seems to work for meinproc.

This change was already applied to the similar code
in update_translations and createdoctemplates.sh
(probably the code should be de-duplicated, but that's
for another day.)