followup #17225: simplify code after removing gc2, generational (#17242)

lib/system/gc2.nim

@@ -7,6 +7,9 @@
 #    distribution, for details about the copyright.
 #
 
+# xxx deadcode, consider removing unless something could be reused.
+
+
 #            Garbage Collector
 #
 # The basic algorithm is an incremental mark

lib/system/mmdisp.nim

@@ -68,9 +68,7 @@ else:
       include "system/cellsets"
     when not leakDetector and not useCellIds and not defined(nimV2):
       sysAssert(sizeof(Cell) == sizeof(FreeCell), "sizeof FreeCell")
-  when compileOption("gc", "v2"):
-    include "system/gc2"
-  elif defined(gcRegions):
+  when defined(gcRegions):
     # XXX due to bootstrapping reasons, we cannot use  compileOption("gc", "stack") here
     include "system/gc_regions"
   elif defined(nimV2) or usesDestructors:

lib/system/sysstr.nim

@@ -304,7 +304,7 @@ proc setLengthSeq(seq: PGenericSeq, elemSize, elemAlign, newLen: int): PGenericS
     when not defined(boehmGC) and not defined(nogc) and
          not defined(gcMarkAndSweep) and not defined(gogc) and
          not defined(gcRegions):
-      when false: # compileOption("gc", "v2"):
+      when false: # deadcode: was used by `compileOption("gc", "v2")`
         for i in newLen..result.len-1:
           let len0 = gch.tempStack.len
           forAllChildrenAux(dataPointer(result, elemAlign, elemSize, i),