lib: sbi: Remove redundant sbi_hsm_hart_started() function

The sbi_hsm_hart_started() function is only used by sbi_hsm_hart_stop()
for checking state of calling HART and current domain assignment.

The atomic_cmpxchg() called by sbi_hsm_hart_stop() will check state of
calling hart anyway and domain assignment can be checked by other domain
function such as sbi_domain_is_assigned_hart().

This means sbi_hsm_hart_started() is redundant and can be removed.

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