variable_watch: Remove undocumented and redundant access type

The ALLOWED_UNKNOWN_VARIABLE_READ_ACCESS access type was switched on an
undocumented variable and its lookup caused an unnecessary performance
impact.  Remove it.

Help/release/dev/variable_watch-no-allowed-access.rst

@@ -0,0 +1,8 @@
+variable_watch-no-allowed-access
+--------------------------------
+
+* Callbacks established by the :command:`variable_watch` command will no
+  longer receive the ``ALLOWED_UNKNOWN_READ_ACCESS`` access type when
+  the undocumented ``CMAKE_ALLOW_UNKNOWN_VARIABLE_READ_ACCESS`` variable is
+  set.  Uninitialized variable accesses will always be reported as
+  ``UNKNOWN_READ_ACCESS``.

Source/cmMakefile.cxx

@@ -2472,19 +2472,8 @@ const char* cmMakefile::GetDefinition(const std::string& name) const
       }
     else
       {
-      // are unknown access allowed
-      const char* allow = this->Internal->VarStack.top()
-        .Get("CMAKE_ALLOW_UNKNOWN_VARIABLE_READ_ACCESS");
-      if(cmSystemTools::IsOn(allow))
-        {
-        vv->VariableAccessed(name,
-          cmVariableWatch::ALLOWED_UNKNOWN_VARIABLE_READ_ACCESS, def, this);
-        }
-      else
-        {
-        vv->VariableAccessed(name,
+      vv->VariableAccessed(name,
           cmVariableWatch::UNKNOWN_VARIABLE_READ_ACCESS, def, this);
-        }
       }
     }
 #endif

Source/cmVariableWatch.cxx

@@ -16,7 +16,6 @@ static const char* const cmVariableWatchAccessStrings[] =
     "READ_ACCESS",
     "UNKNOWN_READ_ACCESS",
     "UNKNOWN_DEFINED_ACCESS",
-    "ALLOWED_UNKNOWN_READ_ACCESS",
     "MODIFIED_ACCESS",
     "REMOVED_ACCESS",
     "NO_ACCESS"

Source/cmVariableWatch.h

@@ -53,7 +53,6 @@ public:
     VARIABLE_READ_ACCESS = 0,
     UNKNOWN_VARIABLE_READ_ACCESS,
     UNKNOWN_VARIABLE_DEFINED_ACCESS,
-    ALLOWED_UNKNOWN_VARIABLE_READ_ACCESS,
     VARIABLE_MODIFIED_ACCESS,
     VARIABLE_REMOVED_ACCESS,
     NO_ACCESS