[build] mkindex.py/grammars.json should be updated, and phased out.

[kaby76]

lab.antlr.org uses grammars.json in this repo. Unfortunately, lab.antlr.org uses the "Antlr interpreter", and any grammars that have actions (e.g., "semantic predicates") will not work. Folks starting out with Antlr will report bugs here and elsewhere for grammars that cannot run. It does not make sense to even offer in lab.antlr.org these grammars. The grammars.json file should be updated to remove all grammars that contain actions.

lab.antlr.org is minimally supported software at this point. antlr/antlr4-lab#69 (comment). The need to build the grammars.json file should be dropped.