[rcore] [web] Simplify EmscriptenResizeCallback()

[asdqwe]

So it doesn't depend on macro'd JS functions. Changes can be tested with:

// Note: usage of `minshell.html` is recommended since it won't force canvas CSS overrides

#include "raylib.h"
int main(void) {
    InitWindow(800, 450, "test");
    SetTargetFPS(60);

    SetWindowState(FLAG_WINDOW_RESIZABLE);

    while (!WindowShouldClose()) {
        BeginDrawing();
        ClearBackground(RAYWHITE);

        DrawText("Resize the browser window manually", 20, 20, 20, BLACK);
        DrawText(TextFormat("canvas size: %i x %i", GetScreenWidth(), GetScreenHeight()), 20, 40, 20, BLACK);

        EndDrawing();
    }
    CloseWindow();
    return 0;
}

[raysan5]

@asdqwe Nice! Thanks for the review!