Remove torbrowser-android-all target

IIRC we added it back then when trying to build standalone fat GeckoView .aar files like so:

./rbm/rbm build geckoview --step merge_aars --target
torbrowser-testbuild --target torbrowser-android-all

However, torbrowser-android-all is not used elsewhere and it seems one can get the same fat .aar file by just doing

./rbm/rbm build geckoview --step merge_aars --target
torbrowser-testbuild --target torbrowser-android-armv7

(or I guess any other Tor Browser Android target)

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