UCD.pm: Don't use CompositionExclusions.txt

UCD.pm: Don't use CompositionExclusions.txt

The motiviation for this patch was to remove dependence of UCD on
another Unicode DB .txt file.

But the subroutine that uses it is out-of-date, now that this property,
and an even more convenient one are accessible from the core.  So the
documentation is also updated to educate people.

Instead of using the file, the routine just uses the core's access
method