build/release/src: remove cbutils/ in srcdir

the builds were being created within that srcdir,
because build/release/src runs lbmk commands within
it, and one of them is building (re-building) it.

there's no point addressing this, other than rm -Rf

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

script/build/release/src

@@ -167,6 +167,7 @@ purge_files()
 	rm -Rf .git .gitignore */.git* coreboot/*/3rdparty/*/.git* \
 	    coreboot/*/util/nvidia/cbootimage/.git* || \
 	    err "purge_files rm-rf2: can't purge .git files/directories"
+	rm -Rf cbutils || err "purge_files: can't remove ${srcdir}/cbutils/"
 	)
 }