Remove requirement for WEBSITE_USE_PLACEHOLDER_DOTNETISOLATED flag

[mattchenderson]

As part of GA for the isolated worker model placeholders, the opt-in flag should no longer be required. The flag should be honored for the explicit disablement state, though.

Setting value	Current behavior	Expected behavior at GA/default
(not provided)	❌ Start without isolated worker model placeholder	✅ Attempt to resolve isolated worker model placeholder
0	❌ Start without isolated worker model placeholder	❌ Start without isolated worker model placeholder
1	✅ Attempt to resolve isolated worker model placeholder	✅ Attempt to resolve isolated worker model placeholder

All other requirements for resolution still apply as before.

[mattchenderson]

This becomes an update to the host to consume configuration injected by the generated worker metadata library. There is a corresponding change needed for the metadata SDK to provide that value: https://github.com/Azure/azure-functions-dotnet-worker/issues/1951

[georgeOsdDev]

Hi @mattchenderson Is this issue still open? What is the actual behavior now? Do I still need to add WEBSITE_USE_PLACEHOLDER_DOTNETISOLATED=1 to app settings?