Since emscripten-core/emscripten#15905 landed
emscripten now includes its own dummy atexit function when building with
EXIT_RUNTIME=0.

This dummy function conflicts with the emscripten-provided one:

```
wasm-ld: error: duplicate symbol: atexit
>>> defined in CMakeFiles/binaryen_wasm.dir/src/binaryen-c.cpp.o
>>> defined in ...wasm32-emscripten/lto/libnoexit.a(atexit_dummy.o)
```

Normally overriding symbols from libc does not causes issues but one
needs to be sure to override all the symbols in a given object file so
that the object in question (atexit_dummy.o) does not get linked in.  In
this case some other symbol being defined in in atexit_dummy.o (e.g.
__cxa_atexit) is likely the cause of the conflict.

Overriding symbols from libc is likely to break in this way as the libc
evolves, and since emscripten is now providing a dummy, just as we want,
its better/safer to simply remove our dummy.

Since emscripten-core/emscripten#15905 landed
emscripten now includes its own dummy atexit function when building with
EXIT_RUNTIME=0.

This dummy function conflicts with the emscripten-provided one:

```
wasm-ld: error: duplicate symbol: atexit
>>> defined in CMakeFiles/binaryen_wasm.dir/src/binaryen-c.cpp.o
>>> defined in ...wasm32-emscripten/lto/libnoexit.a(atexit_dummy.o)
```

Normally overriding symbols from libc does not causes issues but one
needs to be sure to override all the symbols in a given object file so
that the object in question (atexit_dummy.o) does not get linked in.  In
this case some other symbol being defined in in atexit_dummy.o (e.g.
__cxa_atexit) is likely the cause of the conflict.

Overriding symbols from libc is likely to break in this way as the libc
evolves, and since emscripten is now providing a dummy, just as we want,
its better/safer to simply remove our dummy.

After emscripten-core/emscripten#15905 lands Emscripten will no longer use it,
and nothing else needs it AFAIK.

Runtime callbacks can only be functions, not numbers.  In the past we
used to handle C atexit using this mechansim which is why this used to
be needed but that changd in #15905.

Followup to #17303.

The last usage of the func/arg type of callback was removed in #15905
and I don't think we ever advertised this as part of the API.

Runtime callbacks can only be functions, not numbers.  In the past we
used to handle C atexit using this mechansim which is why this used to
be needed but that changd in #15905.

Runtime callbacks can only be functions, not numbers.  In the past we
used to handle C atexit using this mechansim which is why this used to
be needed but that changd in #15905.

Followup to #17303.

The last usage of the func/arg type of callback was removed in #15905
and I don't think we ever advertised this as part of the API.

Followup to #17303.

The last usage of the func/arg type of callback was removed in #15905
and I don't think we ever advertised this as part of the API.