HPCC-11954 Remove unused SIGNAL_TO_EXCEPTION code

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #6271 from richardkchapman/exception-signal-mapping

HPCC-11954 Remove unused SIGNAL_TO_EXCEPTION code

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>