Deprecate NUGET_USE_LEGACY_ASSET_TARGET_FALLBACK_DEPENDENCY_RESOLUTION

[Copilot]

Bug

Fixes: NuGet/Home#14824

Description

NUGET_USE_LEGACY_ASSET_TARGET_FALLBACK_DEPENDENCY_RESOLUTION was added in 6.2 to let users opt back into the pre-6.2 (legacy) dependency resolution behavior for AssetTargetFallback. Telemetry shows 0 usage of the variable, so it's safe to remove.

This PR:

• Removes the environment variable read and the _useLegacyAssetTargetFallbackBehavior field/branching in PackageSpecReferenceDependencyProvider and SourceRepositoryDependencyProvider. The AssetTargetFallback-aware dependency resolution (previously the default unless the variable was set to true) now always runs unconditionally.
• Removes the UsesLegacyAssetTargetFallback telemetry event and its constant from RestoreCommand, per the request on the linked issue to remove the telemetry alongside the variable.
• Updates/removes tests that exercised the legacy env var toggle to reflect the simplified, unconditional behavior.

Clean-up of any now-unused code paths elsewhere is intentionally left as a follow-up, as noted on the issue.

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc. - This was never documented anywhere in https://learn.microsoft.com/en-us/nuget/reference/cli-reference/cli-ref-environment-variables or anywhere else. Telemetry added in March says there is 0% usage.