Revert "riscv: add COMPAT_FREEBSD12 option"

There is no COMPAT_FREEBSD12 in stable/12, only 13 and above.

This reverts commit c31b7fd8b294e64b285898ffbfb613b178948126.

Direct commit to stable/12 since the aforementioned commit was an MFC
that should not have happened.