Remove $user_identity global usage in admin-bar.php

[duck_]

Used by the "My Account" menu, see [16077], the $user_identity global is back compat / 'deprecated' and we shouldn't use it, especially when we now have the full $current_user object via wp_get_current_user() anyway.

See [3566] for "Setup global user vars. Used by set_current_user() for back compat."

Patch also makes use the local $user_id variable instead of re-calling get_current_user_id() again.

[nacin]

In [19284]:

Don't use $user_identity global. props duck_. fixes #19146.