MINOR: pattern: Use reference name as filename to read patterns from a file

MINOR: pattern: Use reference name as filename to read patterns from a file

It is only a small API refactoring. The filename is no longer used when
pat_ref_read_from_file_smp() or pat_ref_read_from_file() functions are
called. The filename was already used to create the reference on the list of
patterns. Thus, we now directly use info from this reference.