gnt-backup import doesn't allow to chose the os

Since it's an import the os is implied to be the one which performed the
export. No -o option is accepted. Reflect this in the manpage.

Reviewed-by: imsnah

man/gnt-backup.sgml

@@ -94,7 +94,6 @@
         <command>import</command>
         <arg choice="req">-n <replaceable>node<optional>:secondary-node</optional></replaceable></arg>
         <arg>-s <replaceable>disksize</replaceable></arg>
-        <arg>-o <replaceable>os-type</replaceable></arg>
         <arg>-m <replaceable>memsize</replaceable></arg>
         <arg>-b <replaceable>bridge</replaceable></arg>
         <sbr>
@@ -130,12 +129,6 @@
         these suffixes map to mebibytes, gibibytes and tebibytes.
       </para>
 
-      <para>
-        The <option>-o</option> options specifies the operating
-        system to be installed. The available operating systems can
-        be listed with <command>gnt-os list</command>.
-      </para>
-
       <para>
         The <option>-m</option> option specifies the memory size for
         the instance, in mebibytes (defaults to 128 MiB).