gnu: bison-3.6: Remove package.

The `bison-3.6' variable is no longer necessary as `bison' itself
already provides this version.

* gnu/packages/bison.scm (bison-3.6): Remove variable.
* gnu/packages/engineering.scm (poke)[native-inputs]: Replace bison-3.6
  with bison.

gnu/packages/bison.scm

@@ -65,17 +65,6 @@ grammar.  It is versatile enough to have many applications, from parsers for
 simple tools through complex programming languages.")
     (license gpl3+)))
 
-(define-public bison-3.6
-  (package
-    (inherit bison)
-    (version "3.6.3")
-    (source
-     (origin
-      (method url-fetch)
-      (uri (string-append "mirror://gnu/bison/bison-" version ".tar.xz"))
-      (sha256
-       (base32 "0gdpnjh6ra9xa9vj6hzjdf0c04x4pjyy8vssm3qdb7fya4v7knq6"))))))
-
 (define-public bison-3.0
   (package
     (inherit bison)

gnu/packages/engineering.scm

@@ -2704,9 +2704,7 @@ GUI.")
       ;; The GUI, which we elide, requires tcl and tk.
       (native-inputs `(("autoconf" ,autoconf)
                        ("automake" ,automake)
-                       ;; Requires bison 3.6+ but we currently only have 3.5.
+                       ("bison" ,bison)
-                       ;; Bison 3.6 will be available in the next core update.
-                       ("bison-3.6" ,bison-3.6)
                        ("clisp" ,clisp)
                        ("dejagnu" ,dejagnu)
                        ("flex" ,flex)