The warning comes from making a function declaration
but never actually defining
it (i.e. giving it a body with statements in). If your program is linking OK, it means this function is not in fact needed anywhere.
My guess is that vcos_timer_delete() has a definition somewhere, but there's some #if nonsense around it which stops it being seen. This may be a bug in the header file, or you may need to #define something to enable the definition.