chore: Drop macos-15

[Kamirus]

Drop building MacOS Intel binaries

[github-actions]

Comparing from 31a7da4 to e47583f:
The produced WebAssembly code seems to be completely unchanged.
In terms of gas, no changes are observed in 5 tests.
In terms of size, no changes are observed in 5 tests.

[Kamirus]

@ggreif wrote:

This needs some handshaking with the SDK release process. Maybe have the MacOS Intel binaries excluded from the upcoming betas of dfx first?

[ggreif]

Here are a few lines that can be deleted too:

  CXX_x86_64-apple-darwin = CXX_aarch64-apple-darwin;
  CXXFLAGS_x86_64-apple-darwin = CXXFLAGS_aarch64-apple-darwin;