x86-64: Remove redundant strcspn-generic/strpbrk-generic/strspn-generic

Remove redundant strcspn-generic, strpbrk-generic and strspn-generic
from sysdep_routines in sysdeps/x86_64/multiarch/Makefile added by

commit c69f960b
Author: Noah Goldstein <goldstein.w.n@gmail.com>
Date:   Sun Jul 3 21:28:07 2022 -0700

    x86: Add support for building str{c|p}{brk|spn} with explicit ISA level

since they have been added to sysdep_routines in sysdeps/x86_64/Makefile.