Remove MediaTranscodingTest

This unit test itself currently uses private apis and does not build
after transcoding is moved to mainline. But since this is being
replaced by the CTS tests in cts/tests/tests/mediatranscoding,
remove it for now.

bug: 174874798
bug: 159172726
Change-Id: I01af32b61b6205f712bbf38d2313faabbc5e9d0d