mk: remove gcc/gnat matching logic (obsolete)

coreboot has this now:

commit c421847fe2fef61586eb313206a4be0f2e491974
Author: Matt DeVillier <matt.devillier@gmail.com>
Date:   Mon Dec 29 10:12:17 2025 -0600

    util/crossgcc: Fix GNAT detection for gnat-15

this one is advisory instead, telling the user to
create such a symlink as we did. this is much less
invasive.

our own check was bad, because it basically silences
what is otherwise an error. coreboot's approach is
more correct, so we will defer to that instead.

note that this means older coreboot (for fam15h) is
no longer covered, but those trees don't use gnat
anyway as libgfxinit is unused on those targets

this lbmk patch removes 61 sloc of dead code, which
makes me very happy.

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/env/init.sh

@@ -171,7 +171,7 @@ xbmk_parent_set_env()
 	xbmk_parent_set_export
 	xbmk_set_version
 
-	remkdir "$xbtmp" "$xbtmp/gnupath" "$xbtmp/xbmkpath"
+	remkdir "$xbtmp" "$xbtmp/xbmkpath"
 
 	xbmk_set_pyver
 	xbmk_set_mirror
@@ -216,7 +216,7 @@ xbmk_parent_set_export()
 		err "cachedir '$XBMK_CACHE' is a file" \
 		    "xbmk_parent_set_export" "$@"
 
-	export PATH="$xbtmp/xbmkpath:$xbtmp/gnupath:$PATH"
+	export PATH="$xbtmp/xbmkpath:$PATH"
 	xbmkpath="$PATH"
 
 	# if "y": a coreboot target won't be built if target.cfg says release=n