refactor(3p/lisp/mime4cl): eliminate use of READ-CHAR in PRINT-MIME-PART

Change-Id: Ibb422d3b6720b782620e262bbd3555b9a879ad65
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12854
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Autosubmit: sterni <sternenseemann@systemli.org>

third_party/lisp/mime4cl/mime.lisp

@@ -880,10 +880,7 @@ returns a MIME-MESSAGE object."
           do (write-char (code-char byte) out)))
       (pathname
        (with-open-file (in body)
-         (loop
-            for c = (read-char in nil)
-            while c
-            do (write-char c out)))))))
+         (redirect-stream in out))))))
 
 (defmethod print-mime-part ((part mime-message) (out stream))
   (flet ((hdr (name)