target/riscv: remove RISCV_FEATURE_EPMP

target/riscv: remove RISCV_FEATURE_EPMP

RISCV_FEATURE_EPMP is always set to the same value as the cpu->cfg.epmp
flag. Use the flag directly.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Reviewed-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Message-ID: <20230222185205.355361-7-dbarboza@ventanamicro.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>