go/ast: remove FuncDecl.IsMethod for Go 1.17

The IsMethod method was added to FuncDecl in the process of working on
support for type parameters, but is now only used in one place. It also
didn't go through the proposal process. Remove it for 1.17.

Also clean up a doc comment that mentioned type parameters.

Fixes #46297

Change-Id: I432bdd626324f613baf059540b7c5436985b2b16
Reviewed-on: https://go-review.googlesource.com/c/go/+/323369


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>