Problem: ZMQ_SOCKET_LIMIT and ZMQ_THREAD_PRIORITY have the same value

Solution: remove documents and tests for ZMQ_THREAD_PRIORITY getter. It
never worked and can never work as it has the same value as a get-only
option ZMQ_SOCKET_LIMIT. It cannot be changed without breaking ABI.
Note that the setter works fine as ZMQ_SOCKET_LIMIT is get-only.

tests/test_ctx_options.cpp

@@ -112,8 +112,6 @@ void test_ctx_thread_opts (void *ctx_)
           ctx_, ZMQ_THREAD_PRIORITY,
           1 /* any positive value different than the default will be ok */);
         assert (rc == 0);
-        rc = zmq_ctx_get (ctx_, ZMQ_THREAD_PRIORITY);
-        assert (rc == 1);
     }