Delete duplicate preference_footer.xml. This duplicate resource breaks the Gradle build.

According to ag/13310367 this file was meant to be moved, but it was copied instead. I think it is safe to delete the duplicate? Please let me know if that's not true.

Test: gradle builds, and hopefully presubmit passes here
Bug: 152086714
Change-Id: I5714a330a659d0bb9a37d433749f60ec330aa27e