Do not export the numerous Emscripten HEAPxy symbols and remove the last remaining...

Do not export the numerous Emscripten HEAPxy symbols and remove the last remaining JS code which references them. They have long-since been replaced by internal equivalents and a specific feature check on one of them (which is also no longer needed) is triggering an exception from Emscripten. More details are in [https://github.com/emscripten-core/emscripten/issues/24656 | Emscripten ticket #24656], noting that this is not an Emscripten bug but an Emscripten behavior change brought it to light.

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