If the DeviceContext is cancelled, we remove it from the list of
tracked DeviceContexts in the MMPluginManager, so that new port
additions happening at the same time don't end up being added to
it.

This should allow an early cancellation of the DeviceContext without
asserting on g_assert (!device_context->extra_probing_time_id) as
there is no chance that a new port addition has reseted the extra
probing time timeout.

    0x00007c1154f16abf (libc.so.6 - pthread_kill.c: 44)	__pthread_kill_implementation
    0x00007c1154ecbf0c (libc.so.6 - raise.c: 26)	raise
    0x00007c1154eb74aa (libc.so.6 - abort.c: 79)	abort
    0x00007c11556c08a9 (libglib-2.0.so.0 - gtestutils.c: 3450)	g_assertion_message
    0x00007c11556c091d (libglib-2.0.so.0 - gtestutils.c: 3476)	g_assertion_message_expr
    0x00005897d1ed6e7b (ModemManager - mm-plugin-manager.c: 805)	device_context_unref
    0x00005897d1ed6ab9 (ModemManager - mm-plugin-manager.c: 163)	common_async_context_free
    0x00005897d1ed5dd6 (ModemManager - mm-plugin-manager.c: 1136)	port_context_run_ready
    0x00007c1155550e86 (libgio-2.0.so.0 - gtask.c: 1309)	g_task_return_now
    0x00007c115554fe50 (libgio-2.0.so.0 - gtask.c: 1378)	g_task_return
    0x00007c115555047b (libgio-2.0.so.0 - gtask.c: 2022)	g_task_return_new_error
    0x00005897d1ed581a (ModemManager - mm-plugin-manager.c: 296)	port_context_complete
    0x00005897d1ed6259 (ModemManager - mm-plugin-manager.c)	plugin_supports_port_ready
    0x00007c1155550e86 (libgio-2.0.so.0 - gtask.c: 1309)	g_task_return_now
    0x00007c1155550eb9 (libgio-2.0.so.0 - gtask.c: 1323)	complete_in_idle_cb
    0x00007c115569c91b (libglib-2.0.so.0 - gmain.c: 3460)	g_main_dispatch
    0x00007c115569c91b (libglib-2.0.so.0 - gmain.c: 4200)	g_main_context_dispatch
    0x00007c115569cc37 (libglib-2.0.so.0 - gmain.c: 4276)	g_main_context_iterate
    0x00007c115569ceb5 (libglib-2.0.so.0 - gmain.c: 4479)	g_main_loop_run
    0x00005897d1e715bf (ModemManager - main.c: 236)	main
    0x00007c1154eb7705 (libc.so.6 - libc_start_call_main.h: 58)	__libc_start_call_main
    0x00007c1154eb77c1 (libc.so.6 - libc-start.c: 360)	__libc_start_main_impl
    0x00005897d1e70f10 (ModemManager + 0x000b3f10)	_start
    0x00007ffd5acc5057

Manually reproducing this issue was challenging, so adding here the
steps taken, for reference. This is with ModemManager running with
--no-udev (i.e. port additions reported via mmcli). The key to
reproduce the problem is the different timing between the port
additions and removals of the three ttyACM ports.

  COUNT=1
  while ((COUNT < 10000)); do
      echo "$COUNT..."
      COUNT=$((COUNT + 1))
      (
          sudo mmcli --report-kernel-event="name=ttyACM0,subsystem=tty,action=add"
          TIMEOUT="0.0$(((RANDOM % 10) + 1))"
          sleep $TIMEOUT
          sudo mmcli --report-kernel-event="name=ttyACM0,subsystem=tty,action=remove"
      ) &
      (
          sudo mmcli --report-kernel-event="name=ttyACM1,subsystem=tty,action=add"
          TIMEOUT="0.0$(((RANDOM % 10) + 1))"
          sleep $TIMEOUT
          sudo mmcli --report-kernel-event="name=ttyACM1,subsystem=tty,action=remove"
      ) &
      (
          sudo mmcli --report-kernel-event="name=ttyACM2,subsystem=tty,action=add"
          TIMEOUT="0.0$(((RANDOM % 10) + 1))"
          sleep $TIMEOUT
          sudo mmcli --report-kernel-event="name=ttyACM2,subsystem=tty,action=remove"
      ) &
      TIMEOUT="0.0$(((RANDOM % 10) + 1))"
      sleep $TIMEOUT
      COUNT=$((COUNT + 1))
  done