avoid using removePathForcibly everywhere, it is unsafe

avoid using removePathForcibly everywhere, it is unsafe

If the temp directory can somehow contain a hard link, it changes the
mode, which affects all other hard linked files. So, it's too unsafe
to use everywhere in git-annex, since hard links are possible in
multiple ways and it would be very hard to prove that every place that
uses a temp directory cannot possibly put a hard link in it.

Added a call to removeDirectoryForCleanup to test_crypto, which will
fix the problem that commit 17b20a24502aee3bfc5683146c3899a233295aea
was intending to fix, with a much smaller hammer.

Sponsored-by: Dartmouth College's Datalad project