lib: sbi_hart: Remove HART available mask and related APIs

The HART available mask and related APIs are now totally redundant
because of more extensive HART state machine implemented by sbi_hsm.

Due to above, we remove HART available mask and related APIs.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>