depend-cleanup.sh: clean up after riscv64 enhanced string function patch set

This ensures an incremental build succeeds after import of
D47275, D46023, D45730, D45693, D46139, D46230, D46047, and D53248.

Approved by: markj (mentor)
MFC after: 1 month
MFC to: stable/15
Differential Revision: https://reviews.freebsd.org/D53485