Fix ns warning: remove is= use

Trivial, but I noticed the following warning when running cljs tests:

```
WARNING: No such namespace:  at line 1 target/generated/test/cljs/schema/core_test.cljs
```

Pretty unclear, but compiling with latest clojurescript gives gives a better
warning:

```
WARNING: Referred macro cemerick.cljs.test/is= does not exist at line 1 target/generated/test/cljs/schema/core_test.cljs
```

`is=` isn't used and removing it from `:use-macros` fixes the warning.