Remove unused functions from null_main.c and null_input.c

Sys_mkdir is suppose to have a capital M, which is also in null_main.c.

code/null/null_input.c

@@ -32,6 +32,3 @@ void IN_Shutdown( void ) {
 void IN_Restart( void ) {
 }
 
-void Sys_SendKeyEvents (void) {
-}
-