depend-cleanup.sh: Clean up the old arm64 memset.S
This has moved from a generated file in objdir to the source tree.
Remove the old file and any .depend files that reference it.
Reviewed by: bapt (via IRC)
Fixes: 41ccf82b29f3 ("libc/aarch64: Use MOPS implementations of memcpy/memmove/memset where availble")
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D54729