RISC-V: nommu_virt: Drop unused SLAB_MERGE_DEFAULT

Our nommu_virt_defconfig set SLOB=y and SLAB_MERGE_DEFAULT=n.  As of
eb52c0fc

 ("mm: Make SLAB_MERGE_DEFAULT depend on SL[AU]B") it's no
longer necessary to set the second, which appears to never have had any
effect for SLOB=y anyway.

This was suggested by savedefconfig.

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