Remove PThread.mainThreadBlock. NFC.

[sbc100]

The main thread pointer is already available via the
_emscripten_main_browser_thread_id native
function.

There is no point in also storing it in JS.

While making this change I noticed that postamble_minimal.js
was also redundantly calling __emscripten_thread_init and
_emscripten_register_main_browser_thread_id.

[kripken]

This looks correct, but it means JS calls into wasm to get the main thread ID. That may be slower, so we should check if none of the calls are on an important code path?

[sbc100]

This looks correct, but it means JS calls into wasm to get the main thread ID. That may be slower, so we should check if none of the calls are on an important code path?

Yes I checked that. Should have mentioned in the PR.

The only two call sites that needed to be updated were from _emscripten_do_pthread_join and from threadProfiler.updateUi.

[kripken]

I see. In theory join is maybe called in a busy place somewhere, but it seems unlikely, I gree.