QMimeDatabase: Fix handling of duplicate mimetype definitions (2/2).

7721c3d27c6a fixed the case where two similar definitions are in the same
directory. This commit fixes the case where two similar definitions are
in different directories, both in the search path (GenericDataLocation).

If the file extension gives us the same mimetype twice, there's no conflict,
i.e. no reason to fallback to determination from contents.

Change-Id: I72c56004b6d5e88964159e53ec160ce8b06c2264
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>