Replace Contract.Assumes and Contract.Assert with Debug.Assert/Debug.Fail (#14136)

Replace Contract.Assumes and Contract.Assert with Debug.Assert/Debug.Fail (#14136)

* Remove use of Contract.Assert and Contract.Assumes in favor of Debug.xx

* Remove dead IA64 blocks

* Remove use of Contract.Requires in favor of Debug.Assert

* Remove Contract.EndContractBlock()

* Dead comments

* Straggler EndcontractBlock

* Remove all Contract.Ensures

* Remove [Pure] attribute and using statements

* Remove using statements for M.D.Contracts.Internal.Contract

* Rmove CA suppressions for Contracts

* Remove M.D.Contracts.Internal stub

* Comments and extra using

* Revert accidentallly removed #if

* Unix build

* Merge Math*

* Fix assert not updated since desktop

* Asserts add no value: remove

* Avoid infinite recursion in StringBuilder assert

* Remove asserts that should be public parameter validation

* Fix comment

* Remove blank line after open curly