[ng] uninstall: reimplement various recipes using more GNU make features

[ng] uninstall: reimplement various recipes using more GNU make features

This change likely introduces subtle semantic changes in corner cases
and tricky situation.  Given the simplifications and improved uniformity
it offers, we consider that completely acceptable.

With this change, the test 't/instmany-python.sh' passes once again.

* lib/am/header-vars.mk (am.uninst.cmd): New private make function.
(am.uninst.cmd.aux): Likewise, used internally by the above (and
to be considered an implementation detail of it).
(am__uninstall_files_from_dir): Delete, no more needed.
* lib/am/data.am: Rewrite the uninstall recipe(s) to take advantage
of $(am.uninst.cmd).
* lib/am/libs.am: Likewise.
* lib/am/lisp.am: Likewise.
* lib/am/mans.am: Likewise.
* lib/am/python.am: Likewise.
* lib/am/progs.am: Likewise.
* lib/am/scripts.am: Likewise.
* lib/am/texinfos.am: Likewise (and also of other unrelated GNU make
features while we are at it).

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>