adb: delete vestigial SHELL_EXIT_NOTIFY_FD.

There exists no path through which a value other than -1 can be written
to the SHELL_EXIT_NOTIFY_FD.

Bug: http://b/74616284
Test: adb_test
Test: adbd_test
Test: python test_device.py
Change-Id: I0630c302ba06bc76917f0445aea75d2dbe1dc865
(cherry picked from commit dcc055af644ce87f1590ab82680f3e0ca4cee54c)
5 files changed