Remove Ubuntu dockerfile for agent #1860

Job	Run time
action-context	1s
setup (operator)	3m 17s
setup (version-upgrade-hook)	2m 18s
setup (readiness-probe)	2m 32s
setup (agent)	2m 42s
setup (e2e)	2m 9s
tests (replica_set, ubi)	6m 11s
tests (replica_set_enterprise_upgrade_4_5, ubi)	17m 48s
tests (replica_set_enterprise_upgrade_5_6, ubi)	19m 4s
tests (replica_set_enterprise_upgrade_6_7, ubi)	19m 23s
tests (replica_set_recovery, ubi)	6m 51s
tests (replica_set_mongod_readiness, ubi)	14m 14s
tests (replica_set_scale, ubi)	9m 32s
tests (replica_set_scale_down, ubi)	9m 24s
tests (replica_set_change_version, ubi)	30m 24s
tests (feature_compatibility_version, ubi)	31m 39s
tests (prometheus, ubi)	8m 59s
tests (replica_set_tls, ubi)	7m 11s
tests (replica_set_tls_recreate_mdbc, ubi)	9m 37s
tests (replica_set_tls_rotate, ubi)	16m 1s
tests (replica_set_tls_rotate_delete_sts, ubi)	9m 15s
tests (replica_set_tls_upgrade, ubi)	10m 13s
tests (statefulset_arbitrary_config, ubi)	5m 44s
tests (statefulset_arbitrary_config_update, ubi)	8m 16s
tests (replica_set_mongod_config, ubi)	5m 54s
tests (replica_set_cross_namespace_deploy, ubi, true)	5m 46s
tests (replica_set_custom_role, ubi)	6m 8s
tests (replica_set_arbiter, ubi)	11m 22s
tests (replica_set_custom_persistent_volume, ubi)	5m 40s
tests (replica_set_mount_connection_string, ubi)	6m 4s
tests (replica_set_mongod_port_change_with_arbiters, ubi)	23m 3s
tests (replica_set_operator_upgrade, ubi)	8m 29s
tests (replica_set_connection_string_options, ubi)	7m 2s
tests (replica_set_x509, ubi)	15m 17s
	5h 47m 30s

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