Remove Twitter support

It does not work. There are currently no known applications that use
it, and we need to get the terms and conditions reviewed by the SFLC
anyway.

We have been removing it from past stable releases, and it is time to
remove it from master too. Will be restored once the situation changes.

See: https://bugzilla.gnome.org/660273
     https://bugzilla.gnome.org/721061