go.sum: purge unused dependencies

Change-Id: I0e75725696c4c5fe7bade91db6169398f7be0a1a
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/166998
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

go.sum

@@ -1,10 +1,3 @@
-github.com/golang/protobuf v1.2.1-0.20181127190454-8d0c54c12466/go.mod h1:Qd/q+1AKNOZr9uGQzbzCmRO6sUih6GTPZv6a1/R87v0=
-github.com/golang/protobuf/v2 v2.0.0-20181127193627-d7e97bc71bcb h1:xAZBrlIILcZ8PEjDVZFkOVURYqYAF9y3+wnJXB/t7I8=
-github.com/golang/protobuf/v2 v2.0.0-20181127193627-d7e97bc71bcb/go.mod h1:MgUD+N3FwzDmj2CdMsT5ap7K7jx+c9cQDQ7fVhmH+Xw=
+github.com/golang/protobuf v1.2.1-0.20190311202412-6ce2da42b4ff/go.mod h1:5kErVJxM6E04eNoOY5G7ivXy1PbRgsxJrBwZ0obu4lQ=
 github.com/google/go-cmp v0.2.1-0.20181101181452-745b8ec83783 h1:wVZ6laEGf86tNDTpR5mxFyFIclJJiXCxuJhcQKnsOHk=
 github.com/google/go-cmp v0.2.1-0.20181101181452-745b8ec83783/go.mod h1:oXzfMopK8JAjlY9xF4vHSVASa0yLyX7SntLO5aqRK0M=
-golang.org/x/net v0.0.0-20180906233101-161cd47e91fd/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
-golang.org/x/sync v0.0.0-20180314180146-1d60e4601c6f/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
-golang.org/x/tools v0.0.0-20180904205237-0aa4b8830f48 h1:PIz+xUHW4G/jqfFWeKhQ96ZV/t2HDsXfWj923rV0bZY=
-golang.org/x/tools v0.0.0-20180904205237-0aa4b8830f48/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
-google.golang.org/genproto v0.0.0-20180831171423-11092d34479b/go.mod h1:JiN7NxoALGmiZfu7CAH4rXhgtRTLTxftemlI0sWmxmc=