Remove Foundation shims

[kkebo]

The only shim that has not been removed is DirectoryEnumerator.

Unfortunately, it needs fts(3) that wasi-libc hasn't supported yet.

References:

• https://github.com/apple/swift-corelibs-foundation/pull/4898/files#diff-dd2c49919954783c5266a5dce970eb43c41de6f017c12ad3474afd038f971c5fR1123
• Remove Foundation shims swift-syntax#6 (comment)

[kkebo]

FYI,

WebAssembly/wasi-libc#522

[kkebo]

swiftlang/swift#81252

wasi-libc in the main branch has been upgraded to version 25. Hopefully we can unlock DirectoryEnumerator by Swift 6.3.