FindOctave: Remove module pending further work

The `FindOctave` module added by commit 170bcb6f (FindOctave: Add
module to find GNU octave, 2018-11-17, v3.14.0-rc1~283^2) has a few
problems in its implementation that need to be worked out before the
module can be included in a CMake release.  These were missed during
review.  Remove the module for now.  It can be restored later with a
fresh review.

Issue: #18991
Status Job ID Name Coverage
  External
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