Remove package.yaml

This reduces duplication between the modified cabal file and
package.yaml.

graphql.cabal

@@ -1,11 +1,5 @@
 cabal-version: 2.2
 
--- This file has been generated from package.yaml by hpack version 0.33.0.
---
--- see: https://github.com/sol/hpack
---
--- hash: dcf3c9aaaaef892c7f75781a0c181c77477fc5bb2757704494c398b99f7fb1c7
-
 name:           graphql
 version:        0.11.1.0
 synopsis:       Haskell GraphQL implementation