[CSSPGO][llvm-profgen] Change default cold threshold for context merging

llvm-profgen uses profile summary based cold threshold to merge and trim cold context profile. This is to strike a good balance between profile size and performance.

We've been using 99.9% as the cutoff to save profile size without affecting performance. This change switch to use 99.9% instead of 99.9999% as default cold threshold cutoff for llvm-profgen.

Redundant switch csprof-cold-thres is also removed and tests cleaned up.

Differential Revision: https://reviews.llvm.org/D103071