chore: remove duplicate stdio.h includes

Author: phaleth

chapter_4/exercise_4_05/math.c

@@ -1,5 +1,4 @@
 #include <stdio.h>
-#include <stdio.h>
 #include <stdlib.h>
 #include <ctype.h>
 #include <math.h>

chapter_4/exercise_4_06/variables.c

@@ -1,5 +1,4 @@
 #include <stdio.h>
-#include <stdio.h>
 #include <stdlib.h>
 #include <ctype.h>
 #include <math.h>

chapter_4/exercise_4_10/calculator.c

@@ -1,5 +1,4 @@
 #include <stdio.h>
-#include <stdio.h>
 #include <stdlib.h>
 #include <ctype.h>
 #include <math.h>