attrib: Avoid passing raw PDU to gatt_discover_char_desc() callback

By centralizing the PDU parsing and validation internally to GATT API,
the callbacks become simpler.
gatttool: Remove unused code

Since commit 6a3a89774c6738df4d5b77636c4ce747b7497ae9 the code
removed here has no sense.
gatt: Avoid passing raw PDU to read characteristic by UUID

By centralizing the PDU parsing and validation internally to GATT API,
the callbacks become simpler.