Remove unused github.com/golang/mock

Signed-off-by: Manjunath A Kumatagi <mkumatag@in.ibm.com>

vendor.conf

@@ -106,7 +106,6 @@ github.com/tonistiigi/fifo 1405643975692217d6720f8b54aeee1bf2cd5cf4
 
 # cluster
 github.com/docker/swarmkit d5232280c510d70755ab11305d46a5704735371a
-github.com/golang/mock bd3c8e81be01eef76d4b503f5e687d2d1354d2d9
 github.com/gogo/protobuf 8d70fb3182befc465c4a1eac8ad4d38ff49778e2
 github.com/cloudflare/cfssl 7fb22c8cba7ecaf98e4082d22d65800cf45e042a
 github.com/google/certificate-transparency d90e65c3a07988180c5b1ece71791c0b6506826e