chore: remove sclf from the tree

SCLF is quite a big utility library (almost 3€ LOC) with limited
portability (CMUCL, SBCL and CLISP to an extent). Continuing to maintain
it is an unnecessary burden, as depot only uses a fraction of it which
is now inlined into the respective users (mime4cl and mblog).

In the future trimming down ex-sclf.lisp may make sense either by
refactoring the code that uses it or by moving interesting utilities
into e.g. klatre.

Change-Id: I2e73825b6bfa372e97847f25c30731a5aad4a1b5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5922
Tested-by: BuildkiteCI
Autosubmit: sterni <sternenseemann@systemli.org>
Reviewed-by: sterni <sternenseemann@systemli.org>

third_party/lisp/mime4cl/package.lisp

@@ -23,13 +23,7 @@
 
 (defpackage :mime4cl
   (:nicknames :mime)
-  (:use :common-lisp :npg :sclf :trivial-gray-streams)
-  ;; this is stuff that comes from SCLF and clashes with CMUCL's EXT
-  ;; package
-  (:shadowing-import-from :sclf
-                          #:process-wait
-                          #:process-alive-p
-                          #:run-program)
+  (:use :common-lisp :npg :mime4cl-ex-sclf :trivial-gray-streams)
   (:import-from :babel :octets-to-string)
   (:import-from :babel-encodings :get-character-encoding)
   (:export #:*lazy-mime-decode*