clean up isNilConversion for seqs, strings (nim-lang#22057)

Author: ringabout

compiler/sigmatch.nim

@@ -1293,8 +1293,6 @@ proc typeRel(c: var TCandidate, f, aOrig: PType,
  result = isSubtype
  else:
  result = isNone
- elif tfNotNil in f.flags and tfNotNil notin a.flags:
- result = isNilConversion
  of tyNil: result = isNone
  else: discard
  of tyOrdinal:
@@ -1392,11 +1390,7 @@ proc typeRel(c: var TCandidate, f, aOrig: PType,
  else: discard
  of tyString:
  case a.kind
- of tyString:
- if tfNotNil in f.flags and tfNotNil notin a.flags:
- result = isNilConversion
- else:
- result = isEqual
+ of tyString: result = isEqual
  of tyNil: result = isNone
  else: discard
  of tyCstring: