ignition-math6: revert swig dependency (#1124)
ignition-math6: revert swig dependency (#1124)

It's causing CI failures, so revert it for now.

Signed-off-by: Steve Peters <19cdd629ec3b4ea4663c77a4169548f370fdc02f@openrobotics.org>
j-rivero/homebrew-simulation,scpeters/homebrew-simulation,osrf/homebrew-simulation