gnu: dovecot-trees: Remove extra build phase.

* gnu/packages/mail.scm (dovecot-trees)[arguments]: Remove unneeded
'autogen phase.

gnu/packages/mail.scm

@@ -1365,12 +1365,7 @@ It supports mbox/Maildir and its own dbox/mdbox formats.")
      `(#:tests? #f ;No tests exist.
        #:configure-flags (list (string-append "--with-dovecot="
                                               (assoc-ref %build-inputs "dovecot")
-                                              "/lib/dovecot"))
-       #:phases
-       (modify-phases %standard-phases
-         (add-after 'unpack 'autogen
-           (lambda _
-             (invoke "sh" "autogen.sh"))))))
+                                              "/lib/dovecot"))))
     (home-page "https://0xacab.org/riseuplabs/trees")
     (synopsis "NaCL-based Dovecot email storage encryption plugin")
     (description