Remove unused function from SuperMemo XML importer.
Fix double newlines in items imported from SuperMemo.

When importing SuperMemo XML files, indicate to the BeautifulSoup
XML parser that the <br> tag is self-closing. This prevents it
from adding a matching </br> for every <br> tag. These extraneous
tags resulted in extra newlines in items imported from SuperMemo.

While we're at it, fix the same bug for a couple of other
self-closing HTML tags.
Add A-factor to E-factor conversion to SuperMemo import.

The SuperMemo XML importer assumed that the A-factors used by
SuperMemo map 1-1 with E-factors (which correspond to Ease in
Anki). This resulted in too large E-factors for imported items.

This change adds an A-factor to E-factor conversion to the
importer.