[docsui] Disable FileDeleteUiTest

This is a test for a corner case and has high failure rate due to
flakiness. There is more noise than signal until it's deflaked.

Bug: 437215252
Test: atest DocumentsUIGoogleTests:com.android.documentsui.FileDeleteUiTest
Flag: EXEMPT test code
Change-Id: Id41c53eef8599224495ea86405b188f9b5042dea