Rollup merge of #137205 - thaliaarchi:remove-wasi-fileext-tell, r=alexcrichton

Remove `std::os::wasi::fs::FileExt::tell`

Following #137165 (Use `tell` for `<File as Seek>::stream_position`), `tell` is now directly exposed via `stream_position`, making `<File as FileExt>::tell` redundant. Remove it.

`std::os::wasi::fs::FileExt::tell` is currently unstable and tracked in rust-lang/rust#71213.

``@rustbot`` ping wasi

Author: Urgau