lib: sbi: Remove MENVCFG hart feature

If a hart implements privileged spec v1.12 (or higher) then we can
safely assume that menvcfg CSR is present and we don't need MENVCFG
as a hart feature.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>