Update tutorial_server_variables.rst (#719)

fix regression introduced in 9881f7fd0d5d947ac8faa5b846c9d7f57efb03e1

doc/tutorial_server_variables.rst

@@ -95,9 +95,6 @@ some examples for their usage.
    UA_NodeId_deleteMembers(&id3); /* free the allocated string */
 
 
-What is UA_NODEID_STRING_ALLOC for?
-
-
 Adding a variable node to the server that contains a user-defined callback
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