make clean: remove redundant removal of STAGING_DIR

Since STAGING_DIR is defined as (package/Makefile.in):

STAGING_SUBDIR = usr/$(GNU_TARGET_NAME)/sysroot
STAGING_DIR    = $(HOST_DIR)/$(STAGING_SUBDIR)

removing HOST_DIR will automatically remove STAGING_DIR. This patch updates
'make clean' based on this knowledge.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>