Batch of minor Flymake cleanup actions agreed to with Stefan

Batch of minor Flymake cleanup actions agreed to with Stefan

Discussed with Stefan, in no particular order

- Remove aliases for symbols thought to be internal to flymake-proc.el
- Don’t need :group in defcustom and defface in flymake.el
- Fix docstring of flymake-make-diagnostic
- Fix docstring of flymake-diagnostic-functions to clarify keywords.
- Mark overlays with just the property ’flymake, not ’flymake-overlay
- Tune flymake-overlays for performance
- Make flymake-mode-on and flymake-mode-off obsolete
- Don’t use hash-table-keys unless necessary.
- Copyright notice in flymake-elisp.

Added some more

- Clarify docstring of flymake-goto-next-error
- Clarify a comment in flymake--run-backend complaining about ert-deftest.
- Prevent compilation warnings in flymake-proc.el
- Remove doctring from obsolete aliases

Now the changelog:

* lisp/progmodes/flymake-elisp.el: Proper copyright notice.

* lisp/progmodes/flymake-proc.el (flymake-warning-re)
(flymake-proc-diagnostic-type-pred)
(flymake-proc-default-guess)
(flymake-proc--get-file-name-mode-and-masks): Move up to
beginning of file to shoosh compiler warnings
(define-obsolete-variable-alias): Delete many obsolete aliases.

* lisp/progmodes/flymake.el (flymake-error-bitmap)
(flymake-warning-bitmap, flymake-note-bitmap)
(flymake-fringe-indicator-position)
(flymake-start-syntax-check-on-newline)
(flymake-no-changes-timeout, flymake-gui-warnings-enabled)
(flymake-start-syntax-check-on-find-file, flymake-log-level)
(flymake-wrap-around, flymake-error, flymake-warning)
(flymake-note): Don't need :group in these defcustom and defface.
(flymake--run-backend): Clarify comment
(flymake-mode-map): Remove.
(flymake-make-diagnostic): Fix docstring.
(flymake--highlight-line, flymake--overlays): Identify flymake
overlays with just ’flymake.
(flymake--overlays): Reverse order of invocation for
cl-remove-if-not and cl-sort.
(flymake-mode-on)
(flymake-mode-off): Make obsolete.
(flymake-goto-next-error, flymake-goto-prev-error): Fix docstring.
(flymake-diagnostic-functions): Clarify keyword arguments in
docstring.

Maybe squash in that one where I remove many obsoletes