[scudo][standalone] Remove the pthread key from the shared TSD

https://reviews.llvm.org/D87420 removed the uses of the pthread key,
but the key itself was left in the shared TSD registry. It is created
on registry initialization, and destroyed on registry teardown.

There is really no use for it now, so we can just remove it.

Differential Revision: https://reviews.llvm.org/D88046