Remove default empty value for image-expires-after

So that we are always forced to provide a value for this parameter
and hopefully always have expiration configured.

.tekton/collector-component-pipeline.yaml

@@ -86,8 +86,7 @@ spec:
     description: Java build
     name: java
     type: string
-  - default: ""
-    description: Image tag expiration time, time values could be something like
+  - description: Image tag expiration time, time values could be something like
       1h, 2d, 3w for hours, days, and weeks, respectively.
     name: image-expires-after
     type: string