kbuild: Remove remaining references to mkdep

Since mkdep is gone, calling it is surely no a good idea anymore.

arch/ppc/Makefile

@@ -117,5 +117,4 @@ archclean:
 
 archmrproper:
 
-archdep: scripts/mkdep
-	$(MAKEBOOT) fastdep
+archdep:

arch/ppc64/Makefile

@@ -67,4 +67,4 @@ archclean:
 archmrproper:
 
 archdep:
-	$(MAKEBOOT) fastdep
+	$(MAKEBOOT) dep

arch/ppc64/boot/Makefile

@@ -121,9 +121,6 @@ imagesize.c: vmlinux.gz
 clean:
 	rm -f piggyback note addnote $(OBJS) zImage zImage.initrd vmlinux.gz no_initrd.o imagesize.c addSystemMap vmlinux.sm addRamDisk vmlinux.initrd vmlinux.sminitrd 
 
-fastdep:
-	$(TOPDIR)/scripts/mkdep *.[Sch] > .depend
-
 dep:
 	$(CPP) $(CPPFLAGS) -M *.S *.c > .depend