Remove obsolete clang18 files for aarch64

This fixes a warning from "make delete-old" about a non-empty directory
on aarch64.

Reviewed by: dim
Approved by: re (cperciva)
Fixes: 0fca6ea1d4ee
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D48904

(cherry picked from commit 09b435d3dc2a591fba08be04d5659d03cce08285)
(cherry picked from commit 9370c2c331255beac5e451c7841e68b2302ebf91)