presets: Remove undocumented non-standard JSON comment support
CMakePresets.json format added in CMake 3.19 supports comments because its
CMakeSettings.json predecessor from VS did. However, JSON comments are non-standard and not supported by all tooling.
CMake 3.19.[0-5] do not document support for the comments, but silently accept them. !5830 (merged) added documentation of the behavior for 3.20, which has not yet been released. Having thought more about this after !5830 (comment 906381), I think we should remove support for comments from the preset formats.