Fix 'go-ts-mode's incorrect docstring inserted for methods

Fix 'go-ts-mode's incorrect docstring inserted for methods

The docstring inserted with go-ts-mode's C-c C-d was incorrectly
prefixed with the receiver "(myStruct).":

    // (myStruct).act
    func (m *myStruct) act () {...}

The above docstring is not correct because the receiver "myStruct"
should not be in the docstring.  This commit fixes the incorrect
behavior.
* lisp/progmodes/go-ts-mode.el (go-ts-mode--defun-name): New
optional argument SKIP-PREFIX.
(go-ts-mode-docstring): Call (go-ts-mode--defun-name t)
instead of (treesit-defun-name).  (Bug#62371)