broadband-modem: don't assert Modem interface skeleton during enabling

There is no guarantee that during an enabling operation the skeleton
for the Modem interface unconditionally exists, e.g. the modem may
have been flagged as invalid and untracked from DBus during the
operation.

Just remove the assertion, because mm_iface_modem_enable() will
already cleanly fail if the skeleton object is not found.

   Thread 0 (crashed),
    0  libc.so.6!__pthread_kill_implementation [pthread_kill.c : 44 + 0x0],
    1  libc.so.6!raise [raise.c : 26 + 0xb],
    2  libc.so.6!abort [abort.c : 79 + 0xa],
    3  libglib-2.0.so.0!g_assertion_message [gtestutils.c : 3450 + 0x5],
    4  libglib-2.0.so.0!g_assertion_message_expr [gtestutils.c : 3476 + 0x14],
    5  ModemManager!enabling_step [mm-broadband-modem.c : 0 + 0x7],
    6  libgio-2.0.so.0!g_task_return_now [gtask.c : 1309 + 0x5],
    7  libgio-2.0.so.0!g_task_return [gtask.c : 1378 + 0x8],
    8  ModemManager!parent_enabling_started_ready [mm-broadband-modem-mbim.c : 3239 + 0xd],
    9  libgio-2.0.so.0!g_task_return_now [gtask.c : 1309 + 0x5],
   10  libgio-2.0.so.0!complete_in_idle_cb [gtask.c : 1323 + 0x5],
   11  libglib-2.0.so.0!g_main_dispatch [gmain.c : 3460 + 0x2],

0 files changed, 0 insertions, 0 deletions