Clean up the unnecessary code in the method UnsharedNameTable#fromUtf

The code, which is listed below, has two repetitive checks and needs to be cleaned up.

location: com.sun.tools.javac.util.UnsharedNameTable#fromUtf
```
        while (element != null) {
            if (element == null) {
                break;
            }
```

we can see the `while (element != null)` have already checked the `element`. So the `if (element == null)` is redundant and unnecessary.