Remove file_contexts.bin from non-A/B OTA package.

This entry only exists in non-A/B full OTA path. However, this file has
never been loaded by recovery - recovery was only reading the one from
the recovery image (which also has been removed by a recent change in
[1]).

[1]: commit e35926e1aff2e6b9b54656bd59c8178e295a1b7e in
platform/bootable/recovery.

Bug: 63538434
Test: `m dist` on angler. file_contexts.bin is gone from full OTA
      package. Sideloading the generated full OTA on angler.
Change-Id: Iec2e2d36599fa1ed499516a3e74f06cd36ade494