[XHarness] Remove not needed ignore file for System.Transactions. (#5457)

Since we support filtering via categories, we do not longer need the
ignore file in these tests.

tests/bcl-test/BCLTests/common-SystemTransactionsTests.ignore

@@ -1,3 +0,0 @@
-# System.NotImplementedException : DTC unsupported, only SinglePhase commit supported for durable resource managers. 
-MonoTests.System.Transactions.EnlistTest.Vol0_Dur1_2PC
-MonoTests.System.Transactions.EnlistTest.Vol0_Dur2