ref(standalone): Remove old processing code

[tobias-wilfert]

Follow up to #5741 #5731 #5703 which moved the processing logic for the respective items to own processors. This now removes the old processing standalone code which was only used by standalone form-data (dropped silently).

Ref: https://linear.app/getsentry/issue/INGEST-824/clean-up

[tobias-wilfert]

The one "new" logic: We don't forward ungrouped but rather error out here.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

Autofix Details

Bugbot Autofix prepared a fix for the issue found in the latest run.

• ✅ Fixed: Standalone FormData now routed to Error pipeline instead of dropped
  • Added explicit handling to silently drop standalone FormData items when there are no event-creating items in the envelope, restoring the previous behavior where they were dropped by process_standalone.

Or push these changes by commenting:

@cursor push cf6934e31a

Preview (cf6934e31a)

diff --git a/relay-server/src/services/processor.rs b/relay-server/src/services/processor.rs
--- a/relay-server/src/services/processor.rs
+++ b/relay-server/src/services/processor.rs
@@ -395,6 +395,10 @@
                     Envelope::from_parts(headers.clone(), standalone_profiles),
                 ));
             }
+
+            // Drop standalone FormData items silently.
+            // FormData should only be processed alongside an event-creating item.
+            envelope.take_items_by(|i| matches!(i.ty(), ItemType::FormData));
         }
 
         // Make sure we create separate envelopes for each `RawSecurity` report.

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[linear-code]

INGEST-824 Clean up

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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