fix(examples): remove now missing tower feature

It was removed in hyperium#2339

Now clippy is unhappy about us requiring it in examples:

```
warning: invalid feature `tower` in required-features of target `tower-client`: `tower` is not present in [features] section
warning: invalid feature `tower` in required-features of target `tower-server`: `tower` is not present in [features] section
```

Author: bobrik

examples/Cargo.toml

@@ -87,12 +87,10 @@ required-features = ["tls-client-auth"]
 [[bin]]
 name = "tower-server"
 path = "src/tower/server.rs"
-required-features = ["tower"]
 
 [[bin]]
 name = "tower-client"
 path = "src/tower/client.rs"
-required-features = ["tower"]
 
 [[bin]]
 name = "multiplex-server"