cmd/compile/internal/typecheck: simplify tcAppend

cmd/compile/internal/typecheck: simplify tcAppend

After CL 418514, Unified IR now always write concrete type of constant
expressions, thus we don't need to handle untyped string anymore.

Change-Id: Ic930ee2418746ca9f4a5122c614e077bef8d7eb4
Reviewed-on: https://go-review.googlesource.com/c/go/+/422041
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>