lib: sbi: Remove redundant sbi_hart_pmp_xyz() functions

The sbi_hart_pmp_get() and sbi_hart_pmp_check_addr() functions
are not used anymore hence this patch removes these functions.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>