Log Message:
ham/gnuradio-core: Drop ninja option

The currently-documented approach to using ninja for cmake builds is
to set "CMAKE_GENERATOR=ninja".  That works fine for gnuradio-core, so
drop the (default-off) option that was a bespoke implementation of the
same thing.

Guess that ~nobody was using this, and that if n were, that them
pruning it and setting CMAKE_GENERATOR=ninja is less total work than
adding compat, maintaining it, and eventually cleaning it up.