Remove FindSubstring as it's really really slow

Use RegexSearch() instead, which is 100 times faster for large strings
(see benchmark results below).

Example benchmark:

    global func TestFindSubstring(int iterations)
    {
    	var result;
    	for (var i = 0; i < iterations; i++)
    	{
    		result = FindSubstring(hamlet, "and");
    	}
    	return result;
    }

    global func TestRegexSearch(int iterations)
    {
    	var result;
    	for (var i = 0; i < iterations; i++)
    	{
    		result = RegexSearch(hamlet, "and");
    	}
    	return result;
    }

    global func RunBenchmark(int iterations)
    {
    	StartScriptProfiler();
    	Log("FindSubstring: %d iterations", iterations);
    	var substr = TestFindSubstring(iterations);
    	Log("RegexSearch: %d iterations", iterations);
    	var regex = TestRegexSearch(iterations);
    	StopScriptProfiler();

    	if (!DeepEqual(regex, substr))
    		Log("Results differ: %v vs %v", substr, regex);
    }

Results:

    FindSubstring: 100 iterations
    RegexSearch: 100 iterations
    Profiler statistics:
    ==============================
    48903ms      Global.FindSubstring
    48903ms      Global.TestFindSubstring
    47979ms      Global.TakeString
    00504ms      Global.TestRegexSearch
    00016ms      Global.PushBack
    ==============================

planet/System.ocg/String.c

@@ -21,20 +21,6 @@ global func TakeString(string str, int begin, int end)
 	return reduced_str;
 }
 
-// Returns a list of positions where the substring was found in the main string.
-global func FindSubstring(string main_str, string sub_str)
-{
-	var positions = [];
-	var sub_length = GetLength(sub_str);
-	var main_length = GetLength(main_str);
-	if (sub_length > main_length)
-		return positions;	
-	for (var index = 0; index <= main_length - sub_length; index++)
-		if (TakeString(main_str, index, index + sub_length) == sub_str)
-			PushBack(positions, index);
-	return positions;
-}
-
 // Converts a char into a string.
 global func CharToString(int char)
 {