[xrd_ssi] Deletes SSI headers and protobuf definitions

xroot_plugins/CMakeLists.txt

xroot_plugins/messages/CMakeLists.txt

xroot_plugins/messages/CtaFrontendApi.hpp

xroot_plugins/messages/cta_admin.proto

xroot_plugins/messages/cta_common.proto

xroot_plugins/messages/cta_eos.proto

xroot_plugins/messages/cta_frontend.proto

xroot_ssi_pb/README.md

xroot_ssi_pb/XrdSsiPbAlert.hpp

xroot_ssi_pb/XrdSsiPbDebug.hpp

xroot_ssi_pb/XrdSsiPbException.hpp

xroot_ssi_pb/XrdSsiPbIStreamBuffer.hpp

xroot_ssi_pb/XrdSsiPbOStreamBuffer.hpp

xroot_ssi_pb/XrdSsiPbRequest.hpp

xroot_ssi_pb/XrdSsiPbRequestProc.hpp

xroot_ssi_pb/XrdSsiPbService.hpp

xroot_ssi_pb/XrdSsiPbServiceClientSide.hpp