riscv: Remove gate area stubs

Since commit a6c19dfe ("arm64,ia64,ppc,s390,sh,tile,um,x86,mm:
remove default gate area"), which predates riscv's inclusion in
Linux by almost three years, the default behavior wrt the gate area
is sane.  Remove riscv's gate area stubs.

Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: linux-riscv@lists.infradead.org
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>