qga/qapi-schema: Drop obsolete note on "unsupported" errors

The note talks about "unsupported" errors and QERR_UNSUPPORTED.  The
former is vague, and the latter makes sense only in C, not in external
interface documentation.  Fortunately, we don't have to address this
anymore: recent merge commit 3b5efc553eb got rid of these errors.
Delete the note.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240911131206.2503035-2-armbru@redhat.com>

qga/qapi-schema.json

@@ -1,15 +1,6 @@
 # *-*- Mode: Python -*-*
 # vim: filetype=python
 
-##
-# = General note concerning the use of guest agent interfaces
-#
-# "unsupported" is a higher-level error than the errors that
-# individual commands might document.  The caller should always be
-# prepared to receive QERR_UNSUPPORTED, even if the given command
-# doesn't specify it, or doesn't document any failure mode at all.
-##
-
 ##
 # = QEMU guest agent protocol commands and structs
 ##