use regex on import to clean up PB generated content

The export to epub creates two things that cause problems on epub import:
1. @see createPartsAndChapters() in export/epub/class-pb-epub201.php line 893. If this div is present again in an export, princexml will generate errors. I'm proposing to strip it out on import. 
2.  @see createPartsAndChapters() in export/epub/class-pb-epub201.php lines 894-896. The chapter number and title are repeated in the body of the html on import. This content is redundant. I'm proposing to strip it out on import.