CTS: Remove redundant test for ".ota" extension

This test was redundant with the
containsAllStockAndroidMappings_mimeToExt() test which is
already testing this, since this mapping is in
frameworks/base/mime/java-res/android.mime.types

We remove this to remove potential confusion for the reader, and
also so if the ".ota" mapping is changed in the android.mime.types
file in the future, this test suite will remain consistent
without modification.

Bug: 136096979
Test: TreeHugger
Change-Id: I754dce421bd172bc271607537d0625bb5f435629