riscv: Remove obsolete rv32_defconfig file

riscv: Remove obsolete rv32_defconfig file

This file is not used since commit 72f045d19f25 ("riscv: Fixup
difference with defconfig"), where it was replaced by the
32-bit.config fragment. Delete the old file to avoid any confusion.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20231121225320.3430550-1-samuel.holland@sifive.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>