feat(3p/lisp/mime4cl): enable compilation with CCL

Only significant implementation specific code at the moment is FILE-SIZE
which isn't very important. We can also easily implement it for CCL.

Additionally, we clean up an unused lexical variable warning and remove
a duplicate definiton of MIME-TYPE-STRING fro MIME-UNKNOWN-PART that CCL
doesn't like.

Change-Id: I7c960e50dcdc1d3e46cb4945f36ea315a3c9838d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12862
Autosubmit: sterni <sternenseemann@systemli.org>
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

third_party/lisp/mime4cl/ex-sclf.lisp

@@ -274,7 +274,8 @@ ELEMENT-TYPE."
 ;; file; just ask politely.
 (defun file-size (pathname)
   #+sbcl (sb-posix:stat-size (sb-posix:stat pathname))
-  #-sbcl (error "nyi"))
+  #+ccl (ccl:file-data-size pathname)
+  #-(or sbcl ccl) (error "nyi"))
 
 ;; LAZY