Drop libmjpegtools-dev B-d & disable mjpegtools plugins

Christopher Obbardrequested to merge
obbardc/gst-plugins-bad1.0:wip/obbardc/fix-mjpeg-bugs into master

mjpegtools is no longer maintained in debian. Drop libmjpegtools-dev from Build-Depends and disable the mpeg2enc, mplex and y4m plugin build so the package builds without the B-d. Remove the corresponding plugins from the library installation list.

Closes: #1123817

Edited by Christopher Obbard

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