SinkBinding matching a subject using labels doesn't update the subject when label is removed

**Describe the bug**
When a SinkBinding is created with a subject using labels
```
kn source binding create bind-event-display --subject Service:serving.knative.dev/v1:foo=bar --sink ksvc:event-display
```
The binding injects expected variables like `K_SINK` into a Knative Service such as
```
kn service create hello --image SOME_IMAGE -l foo=bar
```

When the service is updated to remove the label
```
kn service update hello -l foo-
```
The binding injected variables are still present in the Knative Service
```
kubectl describe ksvc/hello
.
..
Spec:
Container Concurrency: 0
Containers:
Env:
Name: TARGET
Value: World
Name: K_SINK
Value: http://event-display.default.svc.cluster.local
Name: K_CE_OVERRIDES
Image: gcr.io/knative-samples/helloworld-go@sha256:5ea96ba4b872685ff4ddb5cd8d1a97ec18c18fae79ee8df0d29f446c5efe5f50
Name: user-container
```

**Expected behavior**
The binding injected variables are removed when a subject's matching label is no longer present

**To Reproduce**
See steps above
**Knative release version**
v0.26.0

Start by tracing the SinkBinding subject-label matching and reconciliation path, using the reproduction to inspect what happens after the foo label is removed. Done means the binding-injected variables are removed from the Knative Service when the subject no longer matches.

Written by the indexing model from the issue text.