Uwe Brauer <oub@mat.ucm.es> wrote:
> I use supercite

¿Thatʼs a package, that formats citations in mail like that, right:

        UB> I use supercite

If so, let me advise you against using it, and any idiosyncratic citation styles in general.

> But sometimes I want to delte indentation of the quoted text:
>
> So I wrote that small funktion
>
> (defun my-delete-indentation-msg ()
> "Small hack to delete indentention of quotes caused by  `sc-citation-leader'."
>   (interactive)
>   (save-excursion
>     (mail-text)
>     (indent-rigidly (point) (point-max) -50 -50)))
>
> But of course it kills all indentation.

Only that is less than 50 chars wide, IIUC.

> Is there a way only to kill the indentation of the quoted text, which has a
>       >  prefix
>
> Or a 
>
>      RMS> prefix

Why there should not be?  What youʼve just described does not feature anything ‘supercite’-specific and translates to Elisp straightforwardly.  Interpreting your ‘RMS’ as any word (in a sense of syntax tables):

	(defun oub-mail-unindent (&optional beg end)
	  (interactive (when (use-region-p)
	                 (list (region-beginning) (region-end))))
	  (setq beg (or beg (mail-text-start))
	        end (or end (point-max)))
	  (save-excursion
	    (goto-char end) (forward-line 0)
	    (while (<= beg (point))
	      (when (looking-at
	             (rx (seq (one-or-more (literal sc-citation-leader))
	                      (group (zero-or-more word) ">"))))
	        (replace-match "\\1" t))
	      (forward-line -1))))

(Not tested.)