Allow trivially autoloading uses of transient's define macros

Allow trivially autoloading uses of transient's define macros

Since 49e41991b2f transient-define-prefix itself was autoloaded, but
that meant that when ever an autoload file was loaded, which contained
an autoload for a command defined using that macro, transient itself
had to be loaded.

That shouldn't be necessary.  For commands using these macros, an
autoload that is identical to what would have been generated if it
had been defined using defun, works just fine.

* lisp/emacs-lisp/loaddefs-gen.el
(loaddefs-generate--make-autoload): Allow uses of
transient-define-prefix, transient-define-suffix,
transient-define-infix and transient-define-argument
to be autoloaded using just ";;;autoload".

* lisp/transient.el (transient-define-prefix): No longer autoload.