Remove $png and $pngDir from build.pl

The current script throws the following error in Perl v5.18.2 on Arch Linux:

```
$ make
Global symbol "$png" requires explicit package name at scripts/build.pl line 73.
Global symbol "$pngDir" requires explicit package name at scripts/build.pl line 74.
Execution of scripts/build.pl aborted due to compilation errors.
Makefile:110: recipe for target 'build/svg-country-squared-fancy/ad.svg' failed
make: *** [build/svg-country-squared-fancy/ad.svg] Error 255
```

I've removed the $png and $pngDir variables since they aren't used at all.

Cheers!