Remove superfluous strings on tba-android_stringsdtd branch

The current Tor Browser Android translation.git branch seems to contain all Fennec strings from https://hg.mozilla.org/mozilla-central/file/812e732d0928/mobile/android/base/locales/en-US/android_strings.dtd, which is unnecessary because these strings are already continuously updated and translated by Mozilla.

So we should remove https://gitweb.torproject.org/translation.git/tree/en/android_strings.dtd?h=tba-android_stringsdtd

Probably most Tor Browser strings are shared between Desktop and Android. So we should not duplicate these either. We could introduce a .dtd for TBA-only strings. Or we could share .dtd and .properties files between Desktop and Android. Any strings only needed for Desktop can be ignored by Android and vice versa.