std.zig.AstGen: don't call convertToNonTupleLike in structDeclInner

[mateusz834]

At the beginning of structDeclInner there is a special path that handles tuples, we don't have to do anything special here.

[mateusz834]

I also have a feeling that the convertToNonTupleLike makes the code hard to reason about, i removed them in https://github.com/ziglang/zig/compare/ziglang:zig:master...mateusz834:zig:remove-convertToNonTupleLike?expand=1

Let me know what you think about this, i can put these commits also here.