Remove the "glue-cache" option (not the feature!)

This issue is meant to be a reminder about the plan for fully obsoleting the glue-cache option in the 9.19 development cycle. See #2146 (closed) for details - this separate issue was created so that we do not forget to follow through with the removal schedule after the option gets deprecated.

I set the due date for this issue rather arbitrarily, assuming we will be in the 9.19 cycle at that time.