Revert "Allow wish interpreter to be defined with TCLTK_PATH"

This reverts commit e2a1bc6.

Junio rightly pointed out this patch doesn't handle the
`make install` target very well:

Junio C Hamano <junkio@cox.net> writes:
> You should never generate new files in the source tree from
> 'install' target.  Otherwise, the usual pattern of "make" as
> yourself and then "make install" as root would not work from a
> "root-to-nobody-squashing" NFS mounted source tree to local
> filesystem.  You should know better than accepting such a patch.

Makefile

@@ -28,8 +28,6 @@ ifndef V
 	QUIET_BUILT_IN = @echo '   ' BUILTIN $@;
 endif
 
-TCLTK_PATH ?= wish
-
 ifeq ($(findstring $(MAKEFLAGS),s),s)
 QUIET_GEN =
 QUIET_BUILT_IN =
@@ -57,7 +55,6 @@ all:: $(ALL_PROGRAMS)
 
 install: all
 	$(INSTALL) -d -m755 '$(DESTDIR_SQ)$(gitexecdir_SQ)'
-	sed -i .bak -e'1,3s|^exec .* "$$0"|exec '"$(TCLTK_PATH)"' "$$0"|' git-gui && rm git-gui.bak
 	$(INSTALL) git-gui '$(DESTDIR_SQ)$(gitexecdir_SQ)'
 	$(foreach p,$(GITGUI_BUILT_INS), rm -f '$(DESTDIR_SQ)$(gitexecdir_SQ)/$p' && ln '$(DESTDIR_SQ)$(gitexecdir_SQ)/git-gui' '$(DESTDIR_SQ)$(gitexecdir_SQ)/$p' ;)