tests: tgz.mk: remove replaced root-y root-m
tests: tgz.mk: remove replaced root-y root-m

These variables have been replaced by install-y and rootfs-y since
commits:

	255061b tests: mpkg.mk: rename root-y into install-y
	a8eb2fd tests: add rootfs and mpkg_rootfs targets

gazoo74/mpkg,gazoo74/mpkg