Remove legacy GC encodings

[tlively]

It was previously possible to opt in to using the legacy GC opcodes with a build
time flag. Now that WasmGC has shipped and users have migrated to the standard
opcodes, remove the option to use the legacy encodings.

[tlively]

• Remove legacy GC encodings #5874 👈
• Use standard GC encodings by default #5873
• main

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[tlively]

This is meant to be landed after the GC proposal goes to phase 4 and the proposal has shipped by default in browsers that previously supported the legacy encodings. In the time between those two events, users can still choose at Binaryen build time whether they want to target the GC experiment on stable browsers using the legacy encodings or newer pre-release versions of browsers that support the standard encodings.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 42.46%. Comparing base (b671b6c) to head (9118d78).

❗ Current head 9118d78 differs from pull request most recent head d475502. Consider uploading reports for the commit d475502 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5874      +/-   ##
==========================================
- Coverage   48.86%   42.46%   -6.41%     
==========================================
  Files         511      482      -29     
  Lines       78407    74725    -3682     
  Branches    12410    11931     -479     
==========================================
- Hits        38312    31730    -6582     
- Misses      36671    39787    +3116     
+ Partials     3424     3208     -216     

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[tlively]

I'm going to go ahead and land this once the tests have passed.