Cygwin: remove CYGWIN=dos_file_warning option

This option has been disabled long ago and nobody missed it.
Removing drops a bit of unneeded code

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/doc/cygwinenv.xml

@@ -15,12 +15,6 @@ by prefixing with <literal>no</literal>.</para>
 
 <itemizedlist mark="bullet">
 
-<listitem>
-<para><envar>(no)dosfilewarning</envar> - If set, Cygwin will warn the
-first time a user uses an "MS-DOS" style path name rather than a POSIX-style
-path name.  Defaults to off.</para>
-</listitem>
-
 <listitem>
 <para><envar>(no)export</envar> - If set, the final values of these
 settings are re-exported to the environment as <envar>CYGWIN</envar> again.
@@ -131,6 +125,11 @@ explicitly in the open(2) call.
 option has been reported in years.</para>
 </listitem>
 
+<listitem>
+<para><envar>(no)dosfilewarning</envar> - This option had been disabled for
+quite some time and nobody missed it.</para>
+</listitem>
+
 <listitem>
 <para><envar>check_case</envar> - This option has been removed in favor of
 real case sensitivity and the per-mount option "posix=[0|1]".  For more