depend-cleanup: Fix match expression for AArch64 asm files

Some routines (strcmp, strncmp, strlen) switched from an asm
implementation from arm-optimized-routines to a new asm implementation.
The default clean_dep rule matches both the old and new dependency
string, resulting in those routines always being rebuilt. Instead use
"arm-optimized-routines" (i.e., contrib subdir for the previous
implementation) as the matching regex.

Also delete the old generated src files.

Reviewed by: fuz
Sponsored by: The FreeBSD Foundation
Fixes: d57842ed2dad ("depend-cleanup.sh: clean up after AArch64 libc SIMD merge")
Differential Revision: https://reviews.freebsd.org/D48459