Drop use of github.com/gorilla/mux #2950

Job	Run time
build / Build	7m 40s
build-windows / Build	7m 8s
Integration Tests (windows)	1m 39s
Integration Tests (kubeflags)	8m 47s
Integration Tests (etcdsnapshot)	9m 55s
Integration Tests (cacertrotation)	5m 30s
Integration Tests (certrotation)	4m 9s
Integration Tests (secretsencryption)	5m 2s
Integration Tests (longhorn)	5m 41s
Integration Tests (localstorage)	3m 49s
Integration Tests (custometcdargs)	3m 9s
Integration Tests (etcdrestore)	4m 16s
Integration Tests (startup)	12m 41s
Integration Tests (flannelnone)	3m 45s
	1h 23m 11s

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