tools: bpftool: create "uninstall", "doc-uninstall" make targets

Create two targets to remove executable and documentation that would
have been previously installed with `make install` and `make
doc-install`.

Also create a "QUIET_UNINST" helper in tools/scripts/Makefile.include.

Do not attempt to remove directories /usr/local/sbin and
/usr/share/bash-completions/completions, even if they are empty, as
those specific directories probably already existed on the system before
we installed the program, and we do not wish to break other makefiles
that might assume their existence. Do remvoe /usr/local/share/man/man8
if empty however, as this directory does not seem to exist by default.

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>