lsan: remove pthread_detach/join interceptors

They don't seem to do anything useful in lsan.
They are needed only if a tools needs to execute
some custom logic during detach/join, or if it uses
thread registry quarantine. Lsan does none of this.
And if a tool cares then it would also need to intercept
pthread_tryjoin_np and pthread_timedjoin_np, otherwise
it will mess thread states.
Fwiw, asan does not intercept these functions either.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D113920