Remove result merger and namespaced typename hint

[temaEmelyan]

Please make sure you consider the following:

• Add tests that use __typename in queries
• Does this change work with all nadel transformations (rename, type rename, hydration, etc)? Add tests for this.
• Is it worth using hints for this change in order to be able to enable a percentage rollout?
• Do we need to add integration tests for this change in the graphql gateway?
• Do we need a pollinator check for this?

[github-actions]

Test Results

  657 files  +  657    657 suites  +657   51s ⏱️ +51s
2 210 tests +2 210  1 652 ✅ +1 652  558 💤 +558  0 ❌ ±0 
2 218 runs  +2 218  1 660 ✅ +1 660  558 💤 +558  0 ❌ ±0 

Results for commit 529088f. ± Comparison against base commit 58c419e.