cmd/compile/internal/walk: remove litas

cmd/compile/internal/walk: remove litas

Since when its only usage is in maplit, and its body is simple enough to
be inlined directly at the caller side.

Change-Id: Id6b8a9d230d0e1e7f8da8d33bbc0073d3e816fb5
Reviewed-on: https://go-review.googlesource.com/c/go/+/403998
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>