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`.

src/utarray.h

@@ -38,11 +38,6 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #define UTARRAY_UNUSED
 #endif
 
-#ifdef oom
-#error "The name of macro 'oom' has been changed to 'utarray_oom'. Please update your code."
-#define utarray_oom() oom()
-#endif
-
 #ifndef utarray_oom
 #define utarray_oom() exit(-1)
 #endif