dump.c: don't display an ARRAY's ARYLEN field

dump.c: don't display an ARRAY's ARYLEN field

Originally xav_arylen was an AV field and was displayed by sv_dump.
In 2005, this ield was removed, and replaced by PERL_MAGIC_arylen_p
magic when needed.

A side effect of this is that sv_dump on a magical AV adds
PERL_MAGIC_arylen_p magic to the av as a side-effect.
Which is undesirable.

This commit just omits displaying 'ARYLEN =' altogether. Any arylen magic
will already be displayed as part of dumping the AV, so it's redundant.