[xrd_ssi] Removes stable branch from submodule

Temporarily use submodule master until we are ready to commit to CTA master. Then we will restore
submodule updates from stable branch.

.gitmodules

 [submodule "xrootd-ssi-protobuf-interface"]
 	path = xrootd-ssi-protobuf-interface
    url = https://:@gitlab.cern.ch:8443/eos/xrootd-ssi-protobuf-interface.git
-   branch = stable
+#   branch = stable