Clean up the view_diff.html template.
Clean up the view_diff.html template.

This change removes some obsolete variable definitions, fixes indentation on
template tags, and generally cleans things up.

Testing done:
Played around with the diff viewer a bit and checked that things looked right.

Reviewed at https://reviews.reviewboard.org/r/4568/

1tush/reviewboard,KnowNo/reviewboard,reviewboard/reviewboard,custode/reviewboard,bkochendorfer/reviewboard,bkochendorfer/reviewboard,chipx86/reviewboard,davidt/reviewboard,reviewboard/reviewboard,KnowNo/reviewboard,beol/reviewboard,1tush/reviewboard,davidt/reviewboard,1tush/reviewboard,brennie/reviewboard,1tush/reviewboard,custode/reviewboard,sgallagher/reviewboard,custode/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,chipx86/reviewboard,1tush/reviewboard,bkochendorfer/reviewboard,1tush/reviewboard,beol/reviewboard,KnowNo/reviewboard,reviewboard/reviewboard,sgallagher/reviewboard,beol/reviewboard,davidt/reviewboard,1tush/reviewboard,sgallagher/reviewboard,brennie/reviewboard,custode/reviewboard,sgallagher/reviewboard,1tush/reviewboard,1tush/reviewboard,beol/reviewboard,KnowNo/reviewboard,chipx86/reviewboard,bkochendorfer/reviewboard,brennie/reviewboard,brennie/reviewboard,davidt/reviewboard