fix(serviceoffer): drop orphan shared-registration tests

[bussyjd]

Summary

go test ./internal/serviceoffercontroller/ has been failing to build on main since #364 was squash-merged:

controller_test.go:119:2: undefined: applySharedRegistrationStatus
controller_test.go:139:2: undefined: applySharedRegistrationStatus

The squash brought over controller_test.go from 0f7d553 (which introduced both the test and the function) but not the matching function definition in controller.go. Every PR targeting main has been CI-red on this package ever since.

The tests describe a feature that isn't on main

applySharedRegistrationStatus was part of an attempted "aggregate ERC-8004 registration services" direction — non-owner ServiceOffers would inherit the owner's agent ID and tx hash. Current main does the opposite: reconcileRegistrationStatus at internal/serviceoffercontroller/controller.go:568-580 returns Registered=False/SingletonConflict for non-owner offers and deletes their RegistrationRequest.

Restoring the function alone would leave it as dead code. Wiring it into reconcileRegistrationStatus would silently revert the singleton direction. Neither is in scope here — this PR is narrowly about unblocking CI.

Changes

• Remove TestApplySharedRegistrationStatus_NonOwnerUsesSharedAgent (controller_test.go:105-127).
• Remove TestApplySharedRegistrationStatus_WaitsForRoute (controller_test.go:129-144).

Net diff: -41 lines, no production code changes.

Verification

$ go build ./...
$ go test -race ./internal/serviceoffercontroller/
ok  github.com/ObolNetwork/obol-stack/internal/serviceoffercontroller  2.125s

Follow-up if shared registration is desired

A proper PR should add back both:

1. applySharedRegistrationStatus in controller.go (body is in git history at 0f7d553:internal/serviceoffercontroller/controller.go:1224-1252).
2. The caller in reconcileRegistrationStatus that switches behavior for non-owner offers from SingletonConflict to applySharedRegistrationStatus.
3. The two tests this PR removes, restored from git history.

Test plan

• CI green on this PR.
• Other open PRs' CI turns green on the internal/serviceoffercontroller package after this merges.