review(http/upgrade): remove frivolous Unpin bound

https://github.com/linkerd/linkerd2-proxy/pull/3479/files#r1894068885

in `main` this isn't currently pinned, so this was needed to add the `B`
parameter originally in development, but tweaking how we poll the body
(_see lines 70-80, below_) means this bound is indeed frivolous now.

this commit removes an extraneous `Unpin` bound.

Co-authored-by: Scott Fleener <[email protected]>
Signed-off-by: katelyn martin <[email protected]>

linkerd/http/upgrade/src/glue.rs

@@ -53,7 +53,7 @@ pub struct HyperConnectFuture<F> {
 
 impl<B> Body for UpgradeBody<B>
 where
-    B: Body + Unpin,
+    B: Body,
     B::Error: std::fmt::Display,
 {
     type Data = B::Data;