cmd/compile/internal/typecheck: remove some un-used functions

cmd/compile/internal/typecheck: remove some un-used functions

CL 394576 removed FuncBody, which is the only caller of CheckUnused and
CheckReturn. CL 394556 removed all usages of curpkg.

controlLabel was added (un-intentionally?) in Cl 277920 but never used.

Change-Id: I7f47f93b4b9ae9c1a681ff4717920f8d2e7c19c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/405094
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>