Remove unused element from pthread struct. NFC

[sbc100]

This reduces the memory overhead of a pthread from 124 bytes to 100 bytes (~20%) saving.

This change was created mostly by Gemini CLI.

[kripken]

Is this worth increasing the diff vs. musl? Generally there are not many pthreads, so we are talking dozens of bytes, maybe hundreds?

[sbc100]

Is this worth increasing the diff vs. musl? Generally there are not many pthreads, so we are talking dozens of bytes, maybe hundreds?

I was kind of hoping we could add this struct to the TLS area of Wasm Workers.. so that wasm workers could use pthread_mutex_lock etc. I imagined that @juj would like this to be as small as possible to avoid growing the size of the wasm worker allocations, but maybe ~50 bytes per works is not significant?