Remove requirement for WEBSITE_USE_PLACEHOLDER_DOTNETISOLATED flag

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.

Start by locating the handling of WEBSITE_USE_PLACEHOLDER_DOTNETISOLATED in the Azure Functions host and its existing tests. Verify the behavior for an unset value, `0`, and `1`, while preserving all other placeholder-resolution requirements; done means the behavior matches the expected GA table.

Written by the indexing model from the issue text.