fix(developer): keyboard_info merge mobile wrong

The .keyboard_info merge of `platformSupport` would not always give the
right result, for .js keyboards. We have a certain lack of information
in the .js which makes it a little tricky to determine precisely what
the set of supported platforms are. Thus, if we find a .js within a
.kmp, we'll assume it supports mobile; if we find a .js by itself,
we'll assume it supports web. (Thus, a keyboard that builds a .js
is assumed to always support web -- but not necessarily mobile now).

Some keyboards that have incorrectly added a .js into a package, e.g.
sil_cipher_music, will be marked as supporting mobile even though they
don't. The correct fix here is to remove the .js from the package for
that keyboard.