kdelibs: improvements

Removed impurities automatically added
to listings derived from both `XDG_CONFIG_DIRS`
and `XDG_DATA_DIRS` that refers to hardcoded
paths.

This shoud allow us to use `kbuildsycoca4` tool
at build time, generating identical builds of
the database independent on the distribution
nix is based on.

Tests:

 -  Built as part of digikam package which make use
    of this syscoca builder tool at build time to
    generate an immutable database. Everything still
    run fine.