Remove XEmacs compat code from Gnus backends

Remove XEmacs compat code from Gnus backends

* lisp/gnus/mm-view.el (mm-inline-image-xemacs): Remove.
(mm-inline-image): Rename from mm-inline-image-emacs.

* lisp/gnus/mml.el: Remove XEmacs compat code.

* lisp/gnus/nnheader.el: Remove XEmacs compat code.

* lisp/gnus/nnimap.el (nnimap-open-connection-1): Remove
XEmacs compat code.

* lisp/gnus/nnir.el (nnir-run-gmane): Remove XEmacs compat code.

* lisp/gnus/nnmail.el (nnmail-pathname-coding-system): Remove
XEmacs compat code.

* lisp/gnus/nnmairix.el: Remove XEmacs compat code.

* lisp/gnus/nnrss.el: Remove XEmacs compat code.

* lisp/gnus/nntp.el: Remove XEmacs compat code.