xen: remove XEN_SCRUB_PAGES in xen.config

Commit 197ecb38

 ("xen/balloon: add runtime control for scrubbing
ballooned out pages") changed config XEN_SCRUB_PAGES to config
XEN_SCRUB_PAGES_DEFAULT. As xen.config sets 'XEN_BALLOON=y' and
XEN_SCRUB_PAGES_DEFAULT defaults to yes, there is no further need to set
this config in the xen.config file.

Remove setting XEN_SCRUB_PAGES in xen.config, which is without
effect since the commit above anyway.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20220810050712.9539-3-lukas.bulwahn@gmail.com
Signed-off-by: Juergen Gross <jgross@suse.com>