Revert "Update "LICENSE.txt" so that it's recognized as MIT"

[stephentoub]

Reverts #36744

All PRs to dotnet/runtime are now failing with:

error : License file content '/Users/runner/runners/2.169.1/work/1/s/LICENSE.TXT' doesn't match the expected license '/Users/runner/runners/2.169.1/work/1/s/.packages/microsoft.dotnet.arcade.sdk/5.0.0-beta.20280.1/tools/Licenses/MIT.txt'.

cc: @richlander, @terrajobst, @jkotas

[ghost]

Tagging subscribers to this area: @ViktorHofer
Notify danmosemsft if you want to be subscribed.

[stephentoub]

(@ViktorHofer, if changes to LICENSE.TXT can cause builds to fail, we need to ensure at least one relevant CI leg runs when it's changed.)

[safern]

@stephentoub I pushed to this branch if that's OK to add that validation. If we want to keep it as a separate change I can of course put it in a new PR.

[safern]

Feel free to merge in order to unblock PRs if you're OK with me adding the change to run the global build on PRs here.