Twenty Thirteen: Minor js cleanup

[obenland]

I noticed some unused and overused variables in our main js file. With the removal of fixed navbar code from functions.js in r24169, it gives us the chance to optimize it further.

[lancewillett]

JS changes look good.

However, you missed bumping the JS version in wp_enqueue_script() call. I'll bump it in the commit.

[lancewillett]

In 24511:

Twenty Thirteen: functions.js optimizations, props obenland. Fixes #24643.

[lancewillett]

In 24514:

Twenty Thirteen: JS fix, after r24511 changes -- add missing semicolon to end variable definition. See #24643.