playground: clean up environment for goprevplay

Now that goprevplay is deployed, we can see that it's inherited behavior
from gotipplay. Fix that.

Change-Id: I3ea2d53a861d70cad5a3f5764f3b6a64aac40492
Reviewed-on: https://go-review.googlesource.com/c/playground/+/368196
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>

Author: findleyr

app.goprev.yaml

@@ -18,5 +18,3 @@ readiness_check:
 
 env_variables:
   MEMCACHED_ADDR: 'memcached-play-golang:11211'
-  GOTIP: "true"
-