[server] drop system_source_cnx for a simpler internal API
system_source_cnx() can be replaced with source_cnx().
If system_source_cnx() is called with dbms_system_base=False, it is
strictly equivalent to calling source_cnx() with the same source,
privileges and interactivity.
If system_source_cnx() is called with dbms_system_base=True, it is
equivalent to calling source_cnx() with the system database and the
same source, privileges and interactivity.
No use of the default value for 'special_privs' was found in the code.