neon: fix help text for BR2_PACKAGE_NEON_NOXML

Commit 4fbff28b removed the 'depends on' statements for the XML choice.
Therefore, the expat and libxml2 options are always available.  Remove
the cryptic part of help text that tells you to select something if it
is not available.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>