refactor(3p/lisp/mime4cl): port remaining base64 decoding to qbase64

DECODE-BASE64-STREAM-TO-SEQUENCE is the only thing that requires
anything fancy: We read into an adjustable array. Alternative could be
using REDIRECT-STREAM and WITH-OUTPUT-TO-STRING, but that is likely
slower (untested).

Test cases are kept for now to confirm that qbase64 is conforming to our
expectations, but can probably dropped in favor of a few more sample
messages in the test suite.

:START and :END are sadly no longer supported and need to be replaced by
SUBSEQ.

Change-Id: I5928aed7551b0dea32ee09518ea6f604b40c2863
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8586
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
Autosubmit: sterni <sternenseemann@systemli.org>

third_party/lisp/mime4cl/package.lisp

@@ -66,8 +66,6 @@
            #:decode-quoted-printable-string
            #:encode-quoted-printable-stream
            #:encode-quoted-printable-sequence
-           #:decode-base64-stream
-           #:decode-base64-string
            #:encode-base64-stream
            #:encode-base64-sequence
            #:parse-RFC2047-text
@@ -83,7 +81,6 @@
            #:with-input-from-mime-body-stream
            ;; endec.lisp
            #:base64-encoder
-           #:base64-decoder
            #:null-encoder
            #:null-decoder
            #:byte-encoder