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>

arch/riscv/configs/nommu_virt_defconfig

@@ -24,7 +24,6 @@ CONFIG_EXPERT=y
 # CONFIG_VM_EVENT_COUNTERS is not set
 # CONFIG_COMPAT_BRK is not set
 CONFIG_SLOB=y
-# CONFIG_SLAB_MERGE_DEFAULT is not set
 # CONFIG_MMU is not set
 CONFIG_SOC_VIRT=y
 CONFIG_SMP=y