spec: remove need for separate Function production (cleanup)

The EBNF production

	Function = Signature FunctionBody .

was used in FunctionDecl, MethodDecl, and FunctionLit, but only
for the latter it shortened the syntax slightly.

This change "inlines" Function which simplifies FunctionDecl and
MethodDecl and gets rid of the Function production.

This has no impact on the specified language. Also, the Function
production is never referred to by the prose, so it's safe to
remove it from the spec.

Finally, neither go/ast nor go/parser have a representation of
this production via a corresponding node or parse function, so
no possibly valuable documentation is lost, either.

Change-Id: Ia2875d31c6ec2d2079081ef481e50bad4f43c694
Reviewed-on: https://go-review.googlesource.com/91515


Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>