Remove return statement support. #6813

For #6333 we worked on improving syntax auto-indent/dedent (e.g. else). We were able to get dedent mostly right for return (see #6497 (comment)), but weren't able to find a good solution for return followed by a [, (, or { right before the cursor (see examples in #6564). We've determined that it would be better to remove dedent support for return entirely than to have some minor cases where it does the wrong thing.

Later, we will work on sorting out indent/dedent for returnin #6564.