Remove the obsolete reference to /file_contexts.

This file no longer exists:
- /file_contexts has been split into plat_file_contexts and
  nonplat_file_contexts since commit
  b236eb6ca204cefcb926e19bd5682f9dcad4021d (system/sepolicy).
- It was named /file_contexts.bin prior to the split.

'-S file_contexts' is also no longer required by e2fsdroid, since commit
2fff6fb036cbbb6dedd7da3d208b312a9038a5ce (external/e2fsprogs). It will
load the file contexts via libselinux.

Test: Trigger the path by performing a data wipe for converting to FBE.
Change-Id: I179939da409e5c0415ae0ea0bf5ddb23f9e6331e