rm: convert more cases to am__rm_f

rm: convert more cases to am__rm_f

Fixes automake bug https://bugs.gnu.org/10828.

Clean up a few more cases where we were doing `test ... || rm ...` to
avoid calling `rm -f` without arguments by leveraging am__rm_f.  These
were harder to find in the source due to their constructed nature.

The clean programs rules in particular were much more complicated than
they needed to be.  This logic boiled down to two things: delete the
list of programs, and then delete the list without the exeext suffix,
but only if the list of programs is non-empty.

The check-TESTS rule was converted to am__rm_f, but a simplification
was missed where the $list variable is inlined.

* bin/automake.in: Delete test -z logic and always call am__rm_f.
* contrib/check-html.am: Use $(am__rm_f) helper.
* doc/automake.texi: Update examples to match current behavior.
* lib/am/check.am: Inline $list variable.
* lib/am/progs.am: Rewrite rule to use $(am__rm_f).