keysymdef.h: delete four symbolic names that serve no purpose

Ten years ago, in commit 6d4acb0e, these four keysyms were added
to support new compose sequences that then could be typed with the
newish German T3 layout -- or at least, that was the intention.

The commit was in response to https://bugs.freedesktop.org/62189,
right before I retracted the patch after learning that there are no
precomposed characters that use those four symbols as diacritics.

The commit should have been reverted then, but... it lingered and
was forgotten.  No layout in xkeyboard-config uses these symbols
(obviously, as they serve no purpose) and meanwhile the T3 layout
itself has become obsolete [1], so... it's time to clean up.

  [1] https://de.wikipedia.org/wiki/DIN_2137#Neufassung_2018



  "die Belegung T3 wird nicht mehr definiert, da die Erfahrung gezeigt
   hat, dass eine solche Tastatur ohnehin nicht produziert wird"

Signed-off-by: Benno Schulenberg <bensberg@telfort.nl>