Remove some IGNORABLE files from ext/ and lib/

There is surely no point in having any of these modules' MANIFEST files in
the core distribution, but other IGNORABLE files should generally stay in
ext/ and lib/ in case they ever get dual-lived (and likewise for modules in
dist/, which are already dual-lived and some of which include even MANIFEST
files, presumably used to roll CPAN releases from).

However, XS-APItest's README file and DBM_Filter's Changes file offer
nothing useful, so remove them.
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