[JuliaLowering] Don't reintroduce `syntax_flags` after macro expansion (#61712)

In https://github.com/JuliaLang/julia/pull/60733, the macro-facing AST
became
structured like Expr, so an adapter (`est_to_dst`) was added so I
wouldn't need
to make a bunch of changes to desugaring. In many cases, `est_to_dst`
makes
desugaring easier, but in the case of re-introducing syntax flags in
`struct`,
`ncat`, and `nrow`, it's just unnecessary complexity. This change just
desugars
the Expr-like version of these forms.