feat(AIP-128): remove unused guidance (#1009)

There is guidance in AIP-128 that is currently not
relied on by any declarative surface.

This is not a comprehensive removal of all unused behavior,
but it does remove guidance this generally unhelpful.

Specifically:

Delete if existing is not necessary: error codes exist to indicate
if a resource has already been deleted, and can be used by declarative
surfaces instead.

For soft deletes: this must only be used in the case where the
resource cannot re-use the same id. Generally it is a simpler
workflow to simple recreate the resource rather than undelete.

* addressing feedback

switching soft-delete guidance to *should not*, explaining the only
caveats.

Author: toumorokoshi

aip/general/0128.md

@@ -102,11 +102,10 @@ interfaces, due to the focus on automation on top of these resources. This list
 is a comprehensive reference to declarative-friendly guidance in other AIPs:
 
 - Resources **must** use user-settable resource IDs: see AIP-133.
-- Resources **must** permit "create or update": see AIP-134.
-- Resources **should** permit "delete if existing": see AIP-135.
 - Resources **should not** employ custom methods: see AIP-136.
 - Resources **must** use the `Update` method for repeated fields: see AIP-144.
 - Resources **must** include certain standard fields: see AIP-148.
 - Resources **must** have an `etag` field: see AIP-154.
-- Resources **must** provide change validation: see AIP-163.
-- Resources **should** support soft delete: see AIP-164.
+- Resources **should** provide change validation: see AIP-163.
+- Resources **should not** implement soft-delete. If the id cannot be re-used,
+  the resource **must** implement soft-delete and the undelete RPC: see AIP-164

aip/general/0134.md

@@ -209,9 +209,6 @@ The user **must** have the update permissions to call `Update` even with
 `allow_missing` set to `true`. For customers that want to prevent users from
 creating resources using the update method, IAM conditions **should** be used.
 
-**Note:** Declarative-friendly resources ([AIP-128][]) **must** expose the
-`bool allow_missing` field.
-
 ### Etags
 
 An API may sometimes need to allow users to send update requests which are

aip/general/0135.md

@@ -112,9 +112,6 @@ rpc DeleteBook(DeleteBookRequest) returns (google.longrunning.Operation) {
 - Both the `response_type` and `metadata_type` fields **must** be specified
   (even if they are `google.protobuf.Empty`).
 
-**Note:** Declarative-friendly resources (AIP-128) **should** use long-running
-delete.
-
 ### Cascading delete
 
 Sometimes, it may be necessary for users to be able to delete a resource as