remove unused explicit dependency rules for crti/crtn

notes by maintainer:

commit 2f853dd6 added these rules
because the new system for handling arch-provided replacement files
introduced for out-of-tree builds did not apply to the crt tree.

commit 63bcda4d later adapted the
makefile logic so that the crt and ldso trees go through the same
replacement logic as everything else, but failed to remove the
explicit rules that assumed the arch would always provide asm
replacements.

in addition to cleaning things up, removing these spurious rules
allows crti/crtn asm to be omitted by an arch (thereby using the empty
C files instead) if they are not needed.

Makefile

@@ -113,10 +113,6 @@ obj/crt/rcrt1.o: $(srcdir)/ldso/dlstart.c
 
 obj/crt/Scrt1.o obj/crt/rcrt1.o: CFLAGS_ALL += -fPIC
 
-obj/crt/$(ARCH)/crti.o: $(srcdir)/crt/$(ARCH)/crti.s
-
-obj/crt/$(ARCH)/crtn.o: $(srcdir)/crt/$(ARCH)/crtn.s
-
 OPTIMIZE_SRCS = $(wildcard $(OPTIMIZE_GLOBS:%=$(srcdir)/src/%))
 $(OPTIMIZE_SRCS:$(srcdir)/%.c=obj/%.o) $(OPTIMIZE_SRCS:$(srcdir)/%.c=obj/%.lo): CFLAGS += -O3