Remove unnecessary preview checks

The following preview checks are unnecessary because they are standard features now.

Attr.java
```
        allowReifiableTypesInInstanceof =
                Feature.REIFIABLE_TYPES_INSTANCEOF.allowedInSource(source) &&
                (!preview.isPreview(Feature.REIFIABLE_TYPES_INSTANCEOF) || preview.isEnabled());
```

JavacParser.java
```
        this.allowYieldStatement = (!preview.isPreview(Feature.SWITCH_EXPRESSION) || preview.isEnabled()) &&
                Feature.SWITCH_EXPRESSION.allowedInSource(source);
```

Resolve.java
```
        allowYieldStatement = (!preview.isPreview(Feature.SWITCH_EXPRESSION) || preview.isEnabled()) &&
                Feature.SWITCH_EXPRESSION.allowedInSource(source);
```

It is good to remove them.