tests/misc: Drop redundant screenInfo declaration

GCC warned about it:

../test/misc.c:36:19: warning: redundant redeclaration of ‘screenInfo’ [-Wredundant-decls]
   36 | extern ScreenInfo screenInfo;
      |                   ^~~~~~~~~~
In file included from ../test/misc.c:30:
../include/scrnintstr.h:688:29: note: previous declaration of ‘screenInfo’ was here
  688 | extern _X_EXPORT ScreenInfo screenInfo;
      |                             ^~~~~~~~~~

test/misc.c

@@ -33,8 +33,6 @@
 
 #include "tests-common.h"
 
-extern ScreenInfo screenInfo;
-
 static void
 dix_version_compare(void)
 {