arm64, riscv64: Drop arch_string.S.

It was empty and unused on these architectures.

src/system/kernel/lib/arch/arm64/Jamfile

@@ -26,7 +26,6 @@ KernelMergeObject kernel_lib_posix_arch_$(TARGET_ARCH).o :
 	kernel_longjmp_return.c
 	kernel_setjmp_save_sigs.c
 
-	arch_string.S
 	memset.c
 	memcpy.c
 

src/system/kernel/lib/arch/riscv64/Jamfile

@@ -28,7 +28,6 @@ KernelMergeObject kernel_lib_posix_arch_$(TARGET_ARCH).o :
 	kernel_longjmp_return.c
 	kernel_setjmp_save_sigs.c
 
-	#arch_string.S
 	memcpy.c
 	memset.c
 

src/system/libroot/posix/string/arch/arm64/Jamfile

@@ -13,7 +13,6 @@ for architectureObject in [ MultiArchSubDirSetup arm64 ] {
 		SEARCH_SOURCE += [ FDirName $(SUBDIR) $(DOTDOT) generic ] ;
 
 		MergeObject <$(architecture)>posix_string_arch_$(TARGET_ARCH).o :
-			arch_string.S
 			memcpy.c
 			memset.c
 			;

src/system/libroot/posix/string/arch/arm64/arch_string.S

@@ -1,6 +0,0 @@
-/*
- * Copyright 2019, Haiku, Inc. All rights reserved.
- * Distributed under the terms of the MIT License.
- */
-
-#include <asm_defs.h>

src/system/libroot/posix/string/arch/riscv64/arch_string.S

@@ -1,21 +0,0 @@
-/*
-** Copyright 2001, Travis Geiselbrecht. All rights reserved.
-** Distributed under the terms of the NewOS License.
-*/
-
-#include <asm_defs.h>
-
-#warning RISCV64: optimized assembly memcpy
-// for now, a generic C one in:
-// * src/system/kernel/lib/arch/riscv64/Jamfile
-// * src/system/runtime_loader/arch/riscv64/Jamfile
-
-#if 0
-
-/* that should be enough for now */
-
-.align 4
-FUNCTION(memcpy):
-/*	void *memcpy(void *dest, const void *src, size_t count) */
-FUNCTION_END(memcpy)
-#endif