RISC-V: Don't show support for 1.9.1 priv spec

The privileged spec 1.9.1 support was removed since binutils 2.43. The
linker only recognizes it and then reports a warning that it may
conflict with other spec versions.

While the support is removed, binutils should still recognize it, but it
shouldn't be exposed to the user in `disassember-options` help.

Signed-off-by: Marek Pikuła <m.pikula@partner.samsung.com>