Fix: AST to string causes double aliases

[Buroni]

Fixes #1426

During ast.toString(), the function alias is being set in two places: At the yy.FuncValue level and the yy.Select level. This is causing double aliases to be generated for functions when stringifying the AST, e.g. SELECT LENGTH(col) AS length AS length.

This problem would have existed on other tokens such as yy.Literals too, but they seem to use an outdated(?) this.alias which always returns undefined. Updating the yy.Literal code to use this.as causes the problem to appear for literals too, e.g. SELECT col AS c AS c.

My solution is to just remove any alias handling-code from tokens which could represent columns, and let yy.Select handle those aliases as it currently does.

[mathiasrw]

Thank you!