It cleans up old ISP code that is no longer used anymore. The code was used to...

It cleans up old ISP code that is no longer used anymore. The code was used to allow only one ISP split depending on the block size. However, this functionality is not needed anymore after the adoption of JVET-N0308 during the Geneva Meeting in March, which limited the usage of ISP to CUs with a maximum size of 64x64 samples. This clean-up eliminates various unnecessary checks and improves the readability of the code.