Remove =x uses from ISLE, and remove support from the DSL compiler. (#4078)

This is a follow-up on #4074: now that we have the simplified syntax, we
can remove the old, redundant syntax.

cranelift/isle/isle/isle_examples/pass/test4.isle

@@ -20,7 +20,7 @@
     (and
       a
       (Ext1 x)
-      (Ext2 =x)))
+      (Ext2 x)))
   (C #t))
 
 (type Opcode (enum A B C))
@@ -39,4 +39,4 @@
 (decl F (Opcode) u32)
 (rule
  (F _)
- $B)
+ $B)