utarray: Stop making `oom` a synonym for `utarray_oom`

We changed `-Doom=...` to `-Dutarray_oom=...` in commit cfc95dbe,
part of the uthash 2.1.0 release (Dec 2018). Since then, we've had
uthash 2.2.0 (Dec 2020) and 2.3.0 (Feb 2021). I think that's enough
time for people to update their projects. Remove the backward-compatibility
hack `#define utarray_oom oom`.