Remove unused includes from regression tests

This cleanup avoids giving the false impression we would be testing any
functions declared in those header files, which in turn simplifies moving tests
that exercise the internal modelling library to a separate folder.

regression/cbmc/Endianness6/main.c

@@ -1,4 +1,4 @@
-#include <stdio.h>
+#include <assert.h>
 
 int main()
 {

regression/cbmc/Function_Pointer13/main.c

@@ -1,5 +1,3 @@
-#include <stdlib.h>
-#include <stdio.h>
 #include <assert.h>
 
 typedef unsigned int u32;
@@ -17,11 +15,11 @@ u32 myfunc_2(u32 value2){
 }
 
 int main(void){
-myfuncptr fptr=NULL;
+  myfuncptr fptr = 0;
 u32 value;
 
 
-assert(fptr == NULL);
+  assert(fptr == 0);
 
 fptr=myfunc_1;
 value=2;

regression/cbmc/Variadic1/main.c

@@ -1,5 +1,4 @@
 #include <stdarg.h>
-#include <stdio.h>
 #include <assert.h>
 
 // how to do it:

regression/cbmc/byte_update1/main.c

@@ -1,4 +1,3 @@
-#include <stdio.h>
 #include <assert.h>
 
 struct A {

regression/cbmc/byte_update3/main.c

@@ -1,4 +1,3 @@
-#include <stdio.h>
 #include <assert.h>
 
 int main(int argc, char** argv) {

regression/cbmc/byte_update4/main.c

@@ -1,4 +1,3 @@
-#include <stdio.h>
 #include <assert.h>
 
 int main(int argc, char** argv) {

regression/cbmc/byte_update5/main.c

@@ -1,4 +1,3 @@
-#include <stdio.h>
 #include <assert.h>
 
 int main(int argc, char** argv) {

regression/cbmc/complex1/main.c

@@ -1,5 +1,4 @@
 #include <assert.h>
-#include <stdio.h>
 
 int main()
 {

regression/cbmc/dynamic_sizeof1/main.c

@@ -1,4 +1,3 @@
-#include <stdlib.h>
 #include <assert.h>
 
 int main()

regression/cbmc/pointer-extra-checks/main.c

@@ -1,6 +1,3 @@
-#include <assert.h>
-#include <stdlib.h>
-
 int main()
 {
   {

regression/cbmc/va_list3/main.c

@@ -1,4 +1,3 @@
-#include <stdio.h> 
 #include <stdarg.h> 
 #include <assert.h>