Delete unnecessary internal layer of Span globalization helpers (#17890)

Delete unnecessary internal layer of Span globalization helpers (#17890)

This change makes the code both smaller and faster. For example, the following is about 1.4x faster with this change:

```
ReadOnlySpan<char> s1 = "Hello world";
ReadOnlySpan<char> s2 = "world";
for (int i = 0; i < 100000000; i++) s1.EndsWith(s2, StringComparison.OrdinalIgnoreCase);
```

Also, I have ported GetCaseCompareOfComparisonCulture code size optimization from #16434 while I was on it because of it fit well with the rest of the changes.