ucp_request_alloc is stubbed out; documentation incorrect

[aedoq]

The implementation of ucp_request_alloc always returns NULL. If this is the intended behaviour, the function should probably be deprecated. Further, the documented return value cannot be correct, since it has the wrong type:

Returns     Error code as defined by ucs_status_t

This was previously discussed in #5991, where the conclusion was for the application to allocate memory (how we do it right now). It would be a QoL improvement to not have to differentiate between requests allocated by UCX and requests allocated by application code when freeing the memory.

[yosefe]

https://github.com/openucx/ucx/pull/10677