elf_common.h: remove obsolete riscv relocations

These were specified at one time, but later removed.

Reviewed by: jrtc27, imp, emaste
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38174

(cherry picked from commit 7738bb89a3cc70dc83ed4e80ee5160c18a7721f3)