libfiemap: Remove brittle tests.

libfiemap functionality is extensively tested via libsnapshot, gsid, and
fiemap_writer_test. However those tests rely on the configuration of the
device, and libfiemap has fairly different behavior based on whether FBE
or metadata encryption is in use. (The former will use loop devices, the
latter uses dm-linear).

To expand coverage, we added tests with very complicated nesting,
creating fiemaps on a rw partition mounted off a device backed by a
fiemap. Unsurprisingly, this was brittle to begin with, and now
consistently fails due to the intermediate partition not having
sepolicy.

Given the preference toward metadata encryption, and the fact that the
FBE mode will still be properly tested via VTS, let's just remove these
tests.

Bug: 148874852
Test: fiemap_image_test
Change-Id: I7be7d44dea81508505c050ed995b3b78086ad35f