riscv: Remove MACHINE_ARCH-specific remnants of riscv64sf.

• Remove now unnecessary MACHINE_ARCHES definition. The default logic in kern_mib.c works fine now for RISC-V.

• Remove custom sv_machine_arch hook from sysentvec.

Fixes: 1ca12bd927d7 Remove the riscv64sf architecture.
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D40648