qapi: Drop redundant documentation of conditional

Documentation generated for dump-skeys contains

    This command is only supported on s390 architecture.

and

    If
    ~~

    "TARGET_S390X"

The former became redundant in commit 901a34a (qapi: add 'If:'
section to generated documentation) added the latter.  Drop the
former.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240129115008.674248-3-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

qapi/misc-target.json

@@ -237,8 +237,6 @@
 #
 # @filename: the path to the file to dump to
 #
-# This command is only supported on s390 architecture.
-#
 # Since: 2.5
 #
 # Example: