Remove unused Ball code.

I was testing whether Ball-AABB would be faster. Will investigate further in a later
PR: #1686

hoomd/AABB.h

@@ -382,50 +382,6 @@ Ball
         m_center += v;
         }
 
-/** Check if a Ball and AABB overlap
-
-    @param other AABB
-    @returns true when the two shapes overlap, false otherwise
-*/
-DEVICE inline bool overlaps(const AABB& other) const
-    {
-    // https://stackoverflow.com/questions/28343716/sphere-intersection-test-of-aabb
-    LongReal d_squared_min = 0;
-
-    vec3<LongReal> box_min = other.getLower();
-    vec3<LongReal> box_max = other.getUpper();
-
-    if( m_center.x < box_min.x )
-        {
-        d_squared_min += ( m_center.x - box_min.x ) * ( m_center.x - box_min.x );
-        }
-    else if( m_center.x > box_max.x)
-        {
-        d_squared_min += ( m_center.x - box_max.x) * ( m_center.x - box_max.x);
-        }
-
-    if( m_center.y < box_min.y )
-        {
-        d_squared_min += ( m_center.y - box_min.y ) * ( m_center.y - box_min.y );
-        }
-    else if( m_center.y > box_max.y)
-        {
-        d_squared_min += ( m_center.y - box_max.y) * ( m_center.y - box_max.y);
-        }
-
-    if( m_center.z < box_min.z )
-        {
-        d_squared_min += ( m_center.z - box_min.z ) * ( m_center.z - box_min.z );
-        }
-    else if( m_center.z > box_max.z)
-        {
-        d_squared_min += ( m_center.z - box_max.z) * ( m_center.z - box_max.z);
-        }
-
-    return d_squared_min <= m_radius * m_radius;
-    }
-    };
-
 // end group overlap
 /*! @}*/