Bluetooth: HFP_AG: Remove unnecessary lock for feature read access

The lock/unlock for AG/HF feature read access is unnecessary.
For AG feature, it only be changed when creating new AG object.
For HF feature, it only be changed when exchanging feature in SLC
establishment.

Remove the hfp_ag_lock/hfp_ag_unlock for AG/HF feature read access.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>