Delete BazelXMLTestObserverRegistration.

This was used as the principal class when `swift_test` built `MH_BUNDLE`-type binaries, but now that we build standard executables and register the observer manually, it's no longer needed or used.

PiperOrigin-RevId: 662926687

Author: allevato

tools/test_observer/BUILD

@@ -8,7 +8,6 @@ swift_library(
     testonly = 1,
     srcs = [
         "BazelXMLTestObserver.swift",
-        "BazelXMLTestObserverRegistration.swift",
         "Locked.swift",
         "StringInterpolation+XMLEscaping.swift",
         "XUnitTestRecorder.swift",