remove Underscores section

The Underscores subsection states:

> When using properties, instance variables should always be accessed and mutated using `self.`.

This is redundant with, and slightly contradictory to, a line in the Variables section:

> Direct instance variable access should be avoided except in initializer methods (`init`, `initWithCoder:`, etc…), `dealloc` methods and within custom setters and getters.

This is my proposed fix.