py, extmod: Remove include of unnecessary system headers.

py/objfloat.c

@@ -28,7 +28,6 @@
 #include <stdio.h>
 #include <string.h>
 #include <assert.h>
-#include <math.h>
 
 #include "py/nlr.h"
 #include "py/parsenum.h"
@@ -37,6 +36,8 @@
 
 #if MICROPY_PY_BUILTINS_FLOAT
 
+#include <math.h>
+
 #if MICROPY_FLOAT_IMPL == MICROPY_FLOAT_IMPL_FLOAT
 #include "py/formatfloat.h"
 #endif