mkosi: Stop sourcing /etc/makepkg.conf

We don't need this anymore since we now just do delayed evaluation
of $CFLAGS in /etc/makepkg.conf and it imports $CFLAGS into the
script environment which messes everything up.

mkosi.images/system/mkosi.conf.d/10-arch/mkosi.build.chroot

@@ -21,9 +21,6 @@ ln --symbolic "$BUILDDIR" "pkg/$ID/build"
 # Because we run with --noextract we are responsible for making sure the source files appear in src/.
 ln --symbolic . "pkg/$ID/src"
 
-# shellcheck source=/dev/null
-. /etc/makepkg.conf
-
 MKOSI_CFLAGS="-O0 -Wp,-U_FORTIFY_SOURCE"
 if ((LLVM)); then
     # TODO: Remove -fno-sanitize-function when https://github.com/systemd/systemd/issues/29972 is fixed.