RISC-V: defconfigs: Remove redundant CONFIG_POWER_RESET

As of ab7fbad0 ("riscv: Fix unmet direct dependencies built based on
SOC_VIRT") we select CONFIG_POWER_RESET=y along with CONFIG_SOC_VIRT,
which is already in defconfig.  This make setting CONFIG_POWER_RESET in
the defconfigs redundant, so remove it to remain consistent with
savedefconfig.

Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>