Remove submodule dropbox-js.

Because I'm going to fork it and move related patches into the new
repository.

Change-Id: I267ebd1024e847655a7d3d4f38543df8dbd3318f
Signed-off-by: Wei-Chung Wen <morisato3711@gmail.com>
Add new submodule "dropbox-js".

This is the one I forked.

Change-Id: If8589ecb0a88eda59be3f67a96c2b10f2d050dd2
Signed-off-by: Wei-Chung Wen <morisato3711@gmail.com>