compiler: don't generate temporaries for composite literals

compiler: don't generate temporaries for composite literals

We were generating temporaries for composite literals when a
conversion to interface type was required.  However, Cherry's
https://golang.org/cl/176459 changed the compiler to insert explicit
type conversions.  And those explicit type conversions insert the
required temporaries in Type_conversion_expression::do_flatten.
So in practice the composite literal do_flatten methods would never
insert temporaries, as the values they see would always be multi_eval_safe.
So just remove the unnecessary do_flatten methods.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/331691