Bluetooth: HFP HF: Remove unused variable 'buf'

'buf' variable of type struct net_buf was one of the arguments for
finish callback in response to each AT command sent. 'buf' variable
not used in that function. So removed 'buf' variables from the argument
list.

Change-Id: I7d6aa6082f524012ae13a04426915076130ddf69
Signed-off-by: Sathish Narasimman <sathish.narasimman@intel.com>