shared/gatt-db: Make gatt_db_clear call gatt_db_clear_range

This makes gatt_db_clear much simpler while making gatt_db_clear_range
detect full database clear resetting next_handle properly if the
database is empty.

(cherry picked from commit f51cf03f8415198404198f7e1334ded6fdb72db4)

Change-Id: I3156fb9876eeb5a3b01441c81543324aaf301ce8
1 file changed