TBA: Don't ship torbutton xpi

During the packaging stage, we bundle torbutton@torproject.org.xpi, but it's already being included in chrome. We can probably save some bytes in the resulting apk.

 1:43.95 16559236 assets/features/torbutton@torproject.org.xpi (OK)

$ ls obj-x86-linux-android/dist/fennec/features/
torbutton@torproject.org.xpi

I think this is caused by the following in torbutton's moz.build:

FINAL_TARGET_FILES.features['torbutton@torproject.org'] += [
    'src/install.rdf'

]

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