mesh: Remove unused functions from net.c

The folllowing functionsa are not being used anywhere in the code and
have been removed:
mesh_net_flush()
mesh_net_prov_caps_get()
mesh_net_priv_key_get()
mesh_net_priv_key_set()
mesh_net_prov_rand()
mesh_net_prov_uni()
mesh_net_id_uuid_set()
mesh_net_test_addr()
mesh_net_test_mode()
mesh_net_uni_range_set()
mesh_net_uni_range_get()
mesh_net_set_window_accuracy()