[spirv-reader] Remove TODO about nested selections

Remove TOOD about special code for finding badly nested selections.
This is already adequately covered by the dominance-violation checks,
and related tests:

  ClassifyCFGEdges_DomViolation_BeforeIfToSelectionInterior
  ClassifyCFGEdges_DomViolation_BeforeSwitchToSelectionInterior
  ClassifyCFGEdges_DomViolation_BeforeLoopToLoopBodyInterior
  ClassifyCFGEdges_DomViolation_BeforeContinueToContinueInterior
  ClassifyCFGEdges_DomViolation_AfterContinueToContinueInterior

Bug: tint:3
Change-Id: I8c547474d1422a54078d748b39a7d22a500d5d1d
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/21482
Reviewed-by: dan sinclair <dsinclair@google.com>
diff --git a/src/reader/spirv/function.cc b/src/reader/spirv/function.cc
index 7140a0a..b3f0c81 100644
--- a/src/reader/spirv/function.cc
+++ b/src/reader/spirv/function.cc
@@ -479,7 +479,6 @@
   if (!FindIfSelectionInternalHeaders()) {
     return false;
   }
-  // TODO(dneto): FindInvalidNestingBetweenSelections
 
   if (!EmitFunctionVariables()) {
     return false;