Remove unneeded LegacyTextDisplayViewer

As of 3bee601e29ea514f45899b0fb50976a86816c39f, this DisplayViewer
plugin is unnecessary. The reason is due to a change that is part of
d53b3c5e2d0dbdfc431b1a1359f451b2bb1fe359: LegacyUI now implements
SwingUI. So all SwingUI-based DisplayViewer plugins now work in the
LegacyUI automatically, without needing a separate wrapper plugin.