feat: remove Sendable (#41)

After flix/flix#9556 there is no Sendable trait
or `Channel.unsafeSend`. You can just send anything. This PR fixes that

Signed-off-by: Jonathan Lindegaard Starup <[email protected]>

Author: JonathanStarup

test/TestArith.flix

@@ -16,7 +16,7 @@
 
 mod TestArith {
 
-    enum Exp with Eq, ToString, Sendable {
+    enum Exp with Eq, ToString {
         case Num(Int32),
         case Add(Exp, Exp),
         case Sub(Exp, Exp),