Stop emitting javadoc for @removed attributes.

We need the attributes to remain public because people might still be
linking against them, but we don't want them showing up in the
documentation any more. Them showing up in the documentation also had
the side effect that it would accidentally mark the parent class of
attributes as @removed, which was not intended.

Bug: 28663748

Change-Id: I2f6eb09455fddf1086e6b24bc3bea5292e8e32b7