Remove prune package data for .NET Framework

[dsplaisted]

Fixes #50689

After some iteration on when pruning enabled, we ended up with two rules which conflict with each other:

• If any target framework of a multi-targeted project is .NET 10 or higher, enable package pruning for all target frameworks by default.
• Never enable package pruning for .NET Framework by default.

To resolve this, this PR removes the package pruning data for .NET Framework. So pruning will technically be enabled, but won't have any effect. If someone wants to manually opt in to pruning for .NET Framework, they would need to add logic to supply the appropriate PrunePackageReference items.