Remove diff calculation in observe-only reconciliation

[mergenci]

Description of your changes

Fixes crossplane-contrib/provider-upjet-aws#1597.

While importing a DynamoDB Table, in the linked issue, Terraform executes CustomizeDiff functions when Upjet calculates the diff in Observe(). One of the CustomizeDiff functions is a validation function that checks whether the configuration that is subject to diff is valid. Because the configuration is practically empty — it only contains the resource name in the import path — validation fails and returns an error, which in turn causes our Observe() to fail.

I've checked how the behavior of managed reconciler changes for observe-only reconciliation. When Upjet always returns ResourceUpToDate = true, this block executes. Otherwise, when Upjet calculates the diff and returns ResourceUpToDate accordingly, this block executes in case ResourceUpToDate = false. The difference between those two blocks are (1) debug messages and (2) metric records.

The difference doesn't matter for the import path, in which ResourceUpToDate doesn't make much sense. We can consider ResourceUpToDate = true, by definition, which is the case in this PR. The difference might matter in observe-only reconciliation that is not an import operation.

TODO:

• Consider updating Terraform Plugin Framework external client as well.

I have:

• Read and followed Upjet's contribution process.
• Run make reviewable to ensure this PR is ready for review.
• Added backport release-x.y labels to auto-backport this PR if necessary.

How has this code been tested

Manually, using the resource manifest given in the linked issue above.