lib: sbi_pmu: remove mhpm_count field in hart feature

After supporting noncontigous hpm event and counters in opensbi, the
number of hpm counters can be calculated by the mhpm_mask. So this field
is unnecessary and can be removed to save some space.

Signed-off-by: Inochi Amaoto <inochiama@outlook.com>
Reviewed-by: Anup Patel <anup@brainfault.org>