...as the latter was the more flexible replacement for the former for LO-
internal use already.  The only gotcha to watch out for is that unoidl-check
cannot be used to check "naked" .urd files, but only ones where the content has
been moved to /UCR via regmerge.

Leave registry/CustomTarget_regcompare_test.mk around to verify that
unoidl-check behaves the same as did regcompare on those old-format .urd/.rdb
files.

Change-Id: Ic13ede48535bf942126c810d88bac7e4081d984e