Cleanup flag "restore_a11y_shortcut_target_..."

Full flag name:
"restore_a11y_shortcut_target_service"

Note that on line 1869 of AccessibilityManagerServiceTest.java, a
different flag is removed. This flag should have been removed in
http://ag/30554709, but I accidentally removed the flag from this CL
("restore_a11y_shortcut_target_service") instead.

Bug: b/341374402
Test: atest AccessibilityShortcutControllerTest SettingsHelperRestoreTest AccessibilityManagerServiceTest
Flag: EXEMPT flag cleanup
Change-Id: I023fed04527e20f3c547155e70c895b9eebacad3