Low: Build: Revert crm.dtd portion of "Allow build outside of top_srcdir"

This portion wasn't needed as unlike the .rng file, crm.dtd is not
auto-generated, so its in the src hierarchy.

Signed-off-by: Simon Horman <horms@verge.net.au>

Mercurial revision: 8651041799c5eaece6f446d595de879121cad5d8