ref-manual: variables.rst: don't mention the INIT_MANAGER "none" option

As it is mainly used for internal purposes, so not necessarily useful
for regular users.

OE-core "nodistro" uses it as a fallback to the default setting.

(From yocto-docs rev: aa4d4034707d7c74488678e77d4f45d65782b2a7)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
CC: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/variables.rst

@@ -3962,7 +3962,6 @@ system and gives an overview of their function and contents.
       -  ``sysvinit``
       -  ``systemd``
       -  ``mdev-busybox``
-      -  ``none``
 
       With ``sysvinit``, the init manager is set to
       :wikipedia:`SysVinit <Init#SysV-style>`, the traditional UNIX init
@@ -3977,10 +3976,6 @@ system and gives an overview of their function and contents.
       and lightest solution, and probably the best choice for low-end systems
       with a rather slow CPU and a limited amount of RAM.
 
-      With ``none``, the init manager is also set to ``sysvinit``. This is the
-      default setting in OpenEmbedded-Core. This option also selects the
-      :wikipedia:`udev <Udev>` device manager.
-
       More concretely, this is used to include
       ``conf/distro/include/init-manager-${INIT_MANAGER}.inc`` into the global
       configuration. You can have a look at the