Fix linemod compile errors

[woodychow]

• Two files had the same "#ifndef PCL_FEATURES_QUANTIZABLE_MODALITY"
• Two files had the same GradientXY structure defined

[taketwo]

This PointRGB type seems to be a duplicate of the standard RGB type, with an exception that the order of color channels is swapped. Perhaps it would be better to update the algorithm implementation to use the standard type.

[woodychow]

I just realized it too. Let me change that. Thanks.

Edit: I just removed the PointRGB struct. It was not used anywhere in the code as point type is templated in the LineMOD classes.