rtapi: remove support for rtlinux 3.x

No one answered the call for maintainers.  In fact, there's no sign that
rtlinux even exists for modern linux versions.
rtapi: drop support for rtai "24.x"

.. this version of rtai was for kernel 2.4.x, which are no longer
supported.
rtapi: drop support for alternate math libs

ever since we hardcoded the loading of rtai_math years ago, we have
never exercised the code path where we allow the use of diet or glibc
math libraries in kernel space.  Drop the related code.
configure: drop never-used RTAI3_MOD variable

this may once have been used to dynamically list the rtai modules to be
loaded, but this list has been static for a long time.