Makefile: drop the NO_INSTALL variable

Makefile: drop the NO_INSTALL variable

The last user was just removed; There is no longer any need to carry it
around. Should we ever run into a need for it again, it is easy enough
to revert this commit.

It is unlikely, though, that we need `NO_INSTALL` again: as we saw with
the just-removed item, `git-remote-testgit`, we have better locations
to put executables and scripts that we do not want to install, e.g.
a subdirectory in `t/`, or `contrib/`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>