tools: remove golang.org/x/exp/apidiff (#303)

golang.org/x/exp should no longer be needed after #167
diff --git a/go.mod b/go.mod
index 95c93d7..c947b4a 100644
--- a/go.mod
+++ b/go.mod
@@ -4,7 +4,6 @@
 
 require (
 	github.com/golang/protobuf v1.3.3
-	golang.org/x/exp v0.0.0-20190121172915-509febef88a4
 	golang.org/x/lint v0.0.0-20190313153728-d0100b6bd8b3
 	golang.org/x/tools v0.0.0-20190524140312-2c0ae7006135
 	google.golang.org/grpc v1.27.0
diff --git a/go.sum b/go.sum
index 4085870..cb101a3 100644
--- a/go.sum
+++ b/go.sum
@@ -18,7 +18,6 @@
 github.com/google/go-cmp v0.2.0/go.mod h1:oXzfMopK8JAjlY9xF4vHSVASa0yLyX7SntLO5aqRK0M=
 github.com/prometheus/client_model v0.0.0-20190812154241-14fe0d1b01d4/go.mod h1:xMI15A0UPsDsEKsMN9yxemIoYk6Tm2C1GtYGdfGttqA=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
-golang.org/x/exp v0.0.0-20190121172915-509febef88a4 h1:c2HOrn5iMezYjSlGPncknSEr/8x5LELb/ilJbXi9DEA=
 golang.org/x/exp v0.0.0-20190121172915-509febef88a4/go.mod h1:CJ0aWSM057203Lf6IL+f9T1iT9GByDxfZKAQTCR3kQA=
 golang.org/x/lint v0.0.0-20181026193005-c67002cb31c3/go.mod h1:UVdnD1Gm6xHRNCYTkRU2/jEulfH38KcIWyp/GAMgvoE=
 golang.org/x/lint v0.0.0-20190227174305-5b3e6a55c961 h1:GmgasJE571dBGXS7E282h2rIZj+KvCLV8z5I6QXbKNI=
diff --git a/tools.go b/tools.go
index 375b314..5bb0c29 100644
--- a/tools.go
+++ b/tools.go
@@ -25,7 +25,6 @@
 
 import (
 	_ "github.com/golang/protobuf/protoc-gen-go"
-	_ "golang.org/x/exp/cmd/apidiff"
 	_ "golang.org/x/lint/golint"
 	_ "golang.org/x/tools/cmd/goimports"
 	_ "honnef.co/go/tools/cmd/staticcheck"