TextDocument: don't change xml documents content type

if their content type ended in xml ( such as image/svg+xml ),
TextDocument._convertToBaseFormat was resetting the content type to
application/xml.

When doing something like:

  wp = web_page_module.newContent(
    portal_type='Web Page',
    content_type='image/svg+xml',
    text_content=svg
  )

wp.getContentType() was sometimes 'image/svg+xml' sometimes
'application/xml', depending on the order of edit.

product/ERP5/bootstrap/erp5_core/DocumentTemplateItem/portal_components/document.erp5.TextDocument.py

@@ -315,7 +315,6 @@ class TextDocument(CachedConvertableMixin, BaseConvertableFileMixin, TextContent
       try:
         tree = etree.fromstring(text_content)
         text_content = etree.tostring(tree, encoding='utf-8', xml_declaration=True)
-        content_type = 'application/xml'
         message = 'Conversion to base format succeeds'
       except etree.XMLSyntaxError: # pylint: disable=catching-non-exception
         message = 'Conversion to base format without codec fails'