Remove related item510 note

@eshadatta, @ckassel

Hi Esha,
I'm prepping WIPs for the fales/io collection, and ran into some MODS validation errors when transforming 510 fields. Instead of asking you to look at and fix the stylesheet, I wanted to take a whack at figuring it out myself. The cause of the error, per Oxygen, is that `<note>` elements cannot be nested.

I tweaked the stylesheet in the branch attached to this PR. The change is minor, I just commented out the
`<note>` and `</note>` tags around the `relatedItem510` template.  The MODS generated by the modified stylesheet validates against the schema.

I'm not an XSL-master by any stretch, so would you mind taking a look at this PR?
I used GitHub b/c it's easy to see the modifications I made to the XSL and also the differences
in the MODS.

Please let me know if you have any questions.

Thank you for your time!
Joe