Nixed mirrorAngleOrigin because it's not correct.

The correct way to mirror an angle in [-PI, PI] around the x-axis is simply to
negate it, which doesn't seem ceremonious enough to require a utility method.

src/test/java/pythagoras/f/MathUtilTest.java

@@ -30,9 +30,4 @@ public class MathUtilTest
 
         assertEquals(MathUtil.lerpa(3*PI4, -3*PI4, 0.5f), -PI, MathUtil.EPSILON);
     }
-
-    @Test public void testMirrorAngleOrigin() {
-        assertEquals(-PI2, MathUtil.mirrorAngleOrigin(PI2), MathUtil.EPSILON);
-        assertEquals(PI2, MathUtil.mirrorAngleOrigin(-PI2), MathUtil.EPSILON);
-    }
 }