storage: remove the splitTrigger stopgap
storage: remove the splitTrigger stopgap

Should be obsolete with #7860.

```
$ make stressrace PKG=./storage TESTS=TestStoreRangeSplitRaceUninitializedRHS
20213 runs so far, 0 failures, over 20m0s
```

sunya123/cockroach,arjun4084346/cockroach,maxlang/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,maxlang/cockroach,vaibhavarora/cockroach,maxlang/cockroach,glycerine/cockroach,veteranlu/cockroach,vivekmenezes/cockroach,paperstreet/cockroach,cockroachdb/cockroach,sunya123/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,petermattis/cockroach,cockroachdb/cockroach,cockroachdb/cockroach,maxlang/cockroach,vivekmenezes/cockroach,paperstreet/cockroach,sunya123/cockroach,vaibhavarora/cockroach,petermattis/cockroach,vivekmenezes/cockroach,petermattis/cockroach,sunya123/cockroach,vaibhavarora/cockroach,petermattis/cockroach,sunya123/cockroach,vaibhavarora/cockroach,glycerine/cockroach,glycerine/cockroach,cockroachdb/cockroach,arjun4084346/cockroach,cockroachdb/cockroach,paperstreet/cockroach,sunya123/cockroach,arjun4084346/cockroach,petermattis/cockroach,paperstreet/cockroach,paperstreet/cockroach,petermattis/cockroach,arjun4084346/cockroach,glycerine/cockroach,cockroachdb/cockroach,paperstreet/cockroach,veteranlu/cockroach,glycerine/cockroach,maxlang/cockroach,arjun4084346/cockroach,paperstreet/cockroach,maxlang/cockroach,glycerine/cockroach,veteranlu/cockroach,maxlang/cockroach,vivekmenezes/cockroach,glycerine/cockroach,petermattis/cockroach,veteranlu/cockroach,sunya123/cockroach,petermattis/cockroach,maxlang/cockroach,vaibhavarora/cockroach,vaibhavarora/cockroach,vivekmenezes/cockroach,arjun4084346/cockroach,vivekmenezes/cockroach,cockroachdb/cockroach,arjun4084346/cockroach,sunya123/cockroach,veteranlu/cockroach,arjun4084346/cockroach,veteranlu/cockroach,vivekmenezes/cockroach,veteranlu/cockroach,glycerine/cockroach,cockroachdb/cockroach,vivekmenezes/cockroach