vendor: Remove standard package "context" from the configuration.

It doesn't make sense to vendor a standard package. With an empty revision this looks like it was a bogus entry anyway.

Fixes https://github.com/youtube/vitess/issues/2551.

vendor/vendor.json

@@ -32,10 +32,6 @@
 			"revision": "686f0e89858ea78eae54d4b2021e6bfc7d3a30ca",
 			"revisionTime": "2016-12-16T21:27:53Z"
 		},
-		{
-			"path": "context",
-			"revision": ""
-		},
 		{
 			"checksumSHA1": "L4I+Spyy1yw6GbDjnx+Qyj+q9kw=",
 			"path": "github.com/aws/aws-sdk-go/aws",