VS: Fix flag table entry for -Qspectre

The Qspectre option was generated by the script (as opposed to
being added by commit v3.13.0-rc1~4^2 (VS: Add flag table entry for
-Qspectre, 2018-10-08), however the comment was not useful and
there was a separate off option generated that was not in the
old table.

Templates/MSBuild/FlagTables/v141_CL.json

@@ -422,17 +422,10 @@
   {
     "name": "SpectreMitigation",
     "switch": "Qspectre",
-    "comment": "Enabled",
+    "comment": "Spectre mitigations",
     "value": "Spectre",
     "flags": []
   },
-  {
-    "name": "SpectreMitigation",
-    "switch": "",
-    "comment": "Disabled",
-    "value": "false",
-    "flags": []
-  },
   {
     "name": "LanguageStandard",
     "switch": "std:c++14",