Since there is no XHTML import; and this way the expensive "deep"
detection (that calls into all the DLP libs, etc) can be avoided.

Times for a hello world input: 56 -> 23 ms is spent in
Desktop::loadComponentFromURL() (41% of original).

Change-Id: Ia2dec3837cf0c548ae2c5a0ca4d47a57a6cbb92a
Reviewed-on: https://gerrit.libreoffice.org/46387Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>