Prevent dead code warning

The window size variable in ecp_pick_window_size() can take values
4, 5 or 6, but we clamp it not to exceed the value of
MBEDTLS_ECP_WINDOW_SIZE. If that is 6 (default) or higher, the
static analyzer will point out that the test:
w > MBEDTLS_ECP_WINDOW_SIZE always evaluates to false.

This commit removes the test for the cases of the window size
large enough to fit all the potential values of the variable.

library/ecp.c

@@ -2004,8 +2004,10 @@ static unsigned char ecp_pick_window_size( const mbedtls_ecp_group *grp,
      * Make sure w is within bounds.
      * (The last test is useful only for very small curves in the test suite.)
      */
+#if( MBEDTLS_ECP_WINDOW_SIZE < 6 )
     if( w > MBEDTLS_ECP_WINDOW_SIZE )
         w = MBEDTLS_ECP_WINDOW_SIZE;
+#endif
     if( w >= grp->nbits )
         w = 2;