Remove horizontal trimming from libsyntax::parse::comments::strip_doc_comment_decoration

The bug this commit fixes manifests itself when you use the single line doc comments and indented code blocks, e.g. like so:

```rust
///~~~
///{
///   a;
///}
///~~~
```
The resultant markdown code with the current code would look like this:
```
{
a;
}
```

See, e.g., http://static.rust-lang.org/doc/std/iterator.html#example-12.

I also removed the horizontal trimming from the block doc comments too, to be consistent. The unindent_pass in librustdoc does a fine job taking care of leading white space.