Remove mapping with ZFA:0000518.

There are conflicting statements in the available literature regarding
the mammalian equivalent of ZFA's 'griseum centrale'.

Candidates include:

* the 'central gray substance of midbrain' (aka the 'periaqueductal
  gray'), according to Olson et al. 2024
  (doi:10.1016/j.ibror.2017.01.001);
* the 'nucleus incertus', according to Spikol et al. 2024
  (doi:10.7554/eLife.89516.3);
* and the 'central gray substance of medulla', based on the
  classification of the ZFA term.

It is better to simply remove the mapping for now.

src/ontology/uberon-edit.obo

@@ -61071,7 +61071,6 @@ xref: FMA:72595
 xref: HBA:265504914
 xref: neuronames:756 {source="BIRNLEX:2638"}
 xref: UMLS:C0262207 {source="BIRNLEX:2638"}
-xref: ZFA:0000518
 intersection_of: UBERON:0035011 ! central gray substance
 intersection_of: part_of UBERON:0001896 ! medulla oblongata
 relationship: part_of UBERON:0001896 {source="FMA"} ! medulla oblongata