Revert overflow-x: auto on .problem-content divs.

This was added in #1432, but is causing problems.  If an answer rule is
contained in a `position: relative` parent, then the MathQuill toolbar
ends up being positioned correctly, but is contained inside the
`.problem-content` div, and so you need to scroll to the right to see
it. Note that the `.ww-feedback-container` is `position: relative`.  So
any answer inside a div with that class will have this issue.

An example problem for which the problem occurs is:

```
DOCUMENT();

loadMacros('PGstandard.pl', 'PGML.pl', 'parserMultiAnswer.pl', 'PGcourse.pl');

$ma = MultiAnswer(1, 2)->with(singleResult => 1);

BEGIN_PGML
[<
    Enter 1: [_]{$ma}{5}

    Enter 2: [_]{$ma}{5}
>]{ [ class => 'ww-feedback-container ww-fb-align-middle' ] }
END_PGML

ENDDOCUMENT();
```

Note that this is not a problem when using the problem editor, since the
width of the `.problem-content` div is the same as its parent.  So this
will need to be tested in an actual set.

For now the overflow is left for solutions and hints.  Those won't have
answer rules, and thus will also not have the MathQuill toolbar issue.

So a different way to fix the issue with dark text overflowing into the
dark region outside of the problem will need to be found.  Any ideas?

Author: drgrice1

htdocs/js/Problem/problem.scss

@@ -12,7 +12,6 @@
 	border: 1px solid #e3e3e3;
 	border-radius: 4px;
 	box-shadow: inset 0 1px 1px rgba(0, 0, 0, 0.05);
-	overflow-x: auto;
 
 	p {
 		margin-top: 1rem;