Remove IncludesFinderWithGCC

[matthijskooijman]

AFAICS the IncludesFinderWithGCC type, as well as the recipe.preproc.includes in platform.txt that was only used by this type, can be removed. When the include collection switched from parsing dependency files to parsing gcc errors, this code became unused but was left around. To prevent confusion, it seems good to remove it now.

[matthijskooijman]

This also applies to https://github.com/arduino/Arduino/blob/master/hardware/arduino/avr/platform.txt#L85-L86