[Refactoring] Remove unnecessary isVarDeclaration() call

`AggregateDeclaration::fields` is typed as an array of `VarDeclaration`. Therefore, assignment its elements to `Dsymbol *` + invocation of `isVarDeclaration` are merely redundant operations.
Merge pull request #2941 from 9rnsr/refactor_fields

[Refactoring] Remove unnecessary isVarDeclaration() call