Remove MODULE_LICENSE_APACHE2 from DocumentsUI

Repositories containing third-party code need a METADATA and a
license_type. Repositories containing only first-party code no longer
need a MODULE_LICENSE_* file.

Bug: 68860345
Bug: 69058154
Bug: 151953481

Test: no code changes
Change-Id: I5170f8e4f7fc67f75f38c0e4984701a90d8fb1b8