Remove libstagefright_filters.

The only caller was the command-line test tool removed last week. Since
this code contains yet more renderscript, we need to either rewrite it
or remove it, so let's remove it in the absence of callers.

Bug: http://b/256932140
Test: treehugger
Change-Id: Idb46f32d12f11ad8ab5e30572ee470d7491207e9