Remove incorrect assert in trans

As an optimization, once unboxed closures receive their environment by
value if it fits within the size of an `int`.  An assert in this code
path assumed that this would only occur if the environment had no more
than a single free variable in it, but multiple smaller free variables
can easily be packed into the space of an `int`, particularly if any
of them are 0-sized.  The assert can simply be removed.

Closes #18652

src/librustc/middle/trans/closure.rs

@@ -322,7 +322,6 @@ fn load_unboxed_closure_environment<'blk, 'tcx>(
                                          self_type,
                                          "unboxed_closure_env");
         store_ty(bcx, bcx.fcx.llenv.unwrap(), datum.val, self_type);
-        assert!(freevars.len() <= 1);
         datum.val
     } else {
         bcx.fcx.llenv.unwrap()