cleanup: 'capi' unit, no need to set EXP_CLUSTER_RESOURCE_SET

It's been a while now that CAPI usptream manifests have a default value envsubst syntax for $EXP_CLUSTER_RESOURCE_SET (ClusterResourceSet=${EXP_CLUSTER_RESOURCE_SET:=true}):

$ kustomize build kustomize-units/capi | grep EXP_CLUSTER_RESOURCE_SET 
        - --feature-gates=MachinePool=${EXP_MACHINE_POOL:=true},
ClusterResourceSet=${EXP_CLUSTER_RESOURCE_SET:=true},ClusterTopology=${CLUSTER_TOPOLOGY:=false},RuntimeSDK=${EXP_RUNTIME_SDK:=false},MachineSetPreflightChecks=${EXP_MACHINE_SET_PREFLIGHT_CHECKS:=false}

We can clean up our code...