The old internal RTF filter used to call SwTxtNode::SetAttr() without
setting SetAttrMode::NOFORMATATTR, so character attributes which cover
the whole node got converted to paragraph attributes.  The new UNO
filter goes through SwXText::insertTextPortion(), which sets
SetAttrMode::NOFORMATATTR, so this doesn't happen. The result of this is
that when the UI sets a new paragraph style on the text node, then such
character attributes are no longer removed.

Given that in RTF you can't really have character properties on a
paragraph, going back to the document model produced by the old internal
filter doesn't sound like the good direction -- not to mention that
changing SwXText::insertTextPortion() this way would be an implicit API
change.

Fix the problem by tweaking SwEditShell::SetTxtFmtColl() instead, so
that it removes these full-text-node character attributes, too. The
logic in SwTxtNode::RstTxtAttr() can be extended later if necessary to
delete more attributes, but to be on the safe side, just handle the bare
minimum necessary to fix the problem for now.

Change-Id: I5faa3226fc0f9c18e005da185fe0830d8c393256