common/lockdep: drop hash<pthread_t> specialization

common/lockdep: drop hash<pthread_t> specialization

quote from http://en.cppreference.com/w/cpp/utility/hash:

> Each standard library header that declares the template std::hash
> provides enabled specializations of std::hash for std::nullptr
> and all cv-unqualified arithmetic types (including any extended
> integer types), all enumeration types, and all pointer types.

since we've switched to C++17. this full specialization is not necessary
anymore.

Signed-off-by: Kefu Chai <kchai@redhat.com>