git gui Makefile - remove Cygwin modifications

git gui Makefile - remove Cygwin modifications

git-gui's Makefile hardcodes the absolute Windows path of git-gui's libraries
into git-gui, destroying the ability to package git-gui on one machine and
distribute to others. The intent is to do this only if a non-Cygwin Tcl/Tk is
installed, but the test for this is wrong with the unix/X11 Tcl/Tk shipped
since 2012. Also, Cygwin does not support a non-Cygwin Tcl/Tk.

The Cygwin git maintainer disables this code, so this code is definitely
not in use in the Cygwin distribution.

The simplest fix is to just delete the Cygwin specific code,
allowing the Makefile to work out of the box on Cygwin. Do so.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>