RISC-V: K210 defconfigs: Drop redundant MEMBARRIER=n

As of 93917ad5 ("RISC-V: Add support for restartable sequence") we
have support for restartable sequences, which default to enabled.  These
select MEMBARRIER, so disabling it is now redundant.

Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>