Don't take a write lock when freeing an EVP_PKEY

Don't take a write lock when freeing an EVP_PKEY

When freeing the last reference to an EVP_PKEY there is no point in
taking the lock for the key. It is the last reference and is being freed
so must only be being used by a single thread.

This should not have been the source of any contention so its unclear to
what extent this will improve performance. But we should not be locking
when we don't need to.

Partially fixes #20286

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20932)