Drop source dependencies on hooks.c and arena.c

Dependencies on hooks.c and arena.c get auto-computed when generating
malloc.o{,s}.d so there is no need to add them manually.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Reviewed-by: Andreas Schwab <schwab@linux-m68k.org>

malloc/Makefile

@@ -269,10 +269,6 @@ $(objpfx)memusage: memusage.sh
 	    -e 's|@REPORT_BUGS_TO@|$(REPORT_BUGS_TO)|' $^ > $@.new \
 	&& rm -f $@ && mv $@.new $@ && chmod +x $@
 
-
-# Extra dependencies
-$(foreach o,$(all-object-suffixes),$(objpfx)malloc$(o)): arena.c hooks.c
-
 # Compile the tests with a flag which suppresses the mallopt call in
 # the test skeleton.
 $(tests:%=$(objpfx)%.o): CPPFLAGS += -DTEST_NO_MALLOPT