riscv: Cleanup rv32_defconfig

Remove unnecessary configs to make rv32_defconfig have a minimal
difference from the defconfig. CONFIG_ARCH_RV32I selects the
CONFIG_32BIT, so putting it in the file is unnecessary. Also, there is
no need to comment on CONFIG_PORTABLE; it should come from carelessness.

Next rv64ilp32_defconfig would like:
  CONFIG_ARCH_RV64ILP32=y
  CONFIG_NONPORTABLE=y

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Palmer Dabbelt <palmer@rivosinc.com>