Removed 341 unnecessary empty 'return' statements

Python doesn't require `return` statements at the end of functions and methods, and I noticed pdfminer had many such unnecessary `return`s. I went through and removed 341 unnecessary statements. Specifically:

* Removed all `return` statements that were the last statement in a function or method.

* For any `return` statements that were the *only* statement in a function, converted them to a `pass`.

Overall this makes it a simpler, more readable codebase, and it's much more Pythonic.