Remove mapping of 'symmetrical commissure'.

Mapping between FBbt's 'symmetrical commissure' and Uberon's 'nervous
system commissure' causes some problem when applying taxon constraints,
due to the Uberon term being linked to the concept of white matter.

Remove the mapping as per Chris Mungall's suggestion in
obophenotype/uberon#325.

src/ontology/mappings.tsv

@@ -121,7 +121,6 @@ FBbt:00005061	UBERON:0001011	exact	hemolymph
 FBbt:00005093	UBERON:0001016	exact	nervous system
 FBbt:00005094	UBERON:0001017	exact	central nervous system
 FBbt:00005100	UBERON:0001018	exact	tract
-FBbt:00005103	UBERON:0001020	exact	symmetrical commissure	See Uberon ticket #325
 FBbt:00005105	UBERON:0001021	exact	nerve
 FBbt:00005136	UBERON:0001027	exact	sensory nerve
 FBbt:00007692	UBERON:0001032	exact	sensory system