Line*.hh: remove unneeded use of static variable

[scpeters]

🦟 Bug fix

Fixes #269.

Summary

From #269 (comment):

There are still some destructor-only fiascos. Hopefully the compiler optimizes it away, but I don't think its guaranteed:

./include/gz/math/Line2.hh:183:        static math::Vector2<T> ignore;
./include/gz/math/Line3.hh:266:        static math::Vector3<T> ignore;

The fix for those last two is easy, in any case. Just remove the static. It's way slower to use "static" anyway. A dummy on the stack should be free. A dummy as a mutable global requires checking an initialization lock every time we call that function.

This removes the static keyword from those variables.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
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• All tests passed (See test coverage)
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