Cleanup Doxygen::terminating

Variable was introduced in doxygen 1.9.4 and used in in doxygen.h in the function `NonTerminatingDeleter`  "Deleter that only deletes an object if doxygen is not already terminating"
It was removed in doxygen 1.8.8 with:
```
Commit: bcc35bd [bcc35bd]
Date: Sunday, May 21, 2023 3:02:22 PM

Refactoring: replace DotRunnerQueue and DotWorkerThread by ThreadPool
```
though the variable `Doxygen::terminating` remained and was only set but not used.
It was used in the past in dot.h in `using DotWorkerThreadPtr = std::unique_ptr< DotWorkerThread, NonTerminatingDeleter<DotWorkerThread > >;`

Author: albert-github

src/doxygen.cpp

@@ -168,7 +168,6 @@ bool                  Doxygen::generatingXmlOutput = FALSE;
 DefinesPerFileList    Doxygen::macroDefinitions;
 bool                  Doxygen::clangAssistedParsing = FALSE;
 QCString              Doxygen::verifiedDotPath;
-volatile bool         Doxygen::terminating = false;
 InputFileEncodingList Doxygen::inputFileEncodingList;
 std::mutex            Doxygen::countFlowKeywordsMutex;
 std::mutex            Doxygen::addExampleMutex;
@@ -11450,7 +11449,6 @@ static void stopDoxygen(int)
   }
   killpg(0,SIGINT);
   cleanUpDoxygen();
-  Doxygen::terminating=true;
   exitTracing();
   exit(1);
 }

src/doxygen.h

@@ -127,7 +127,6 @@ class Doxygen
     static DefinesPerFileList        macroDefinitions;
     static bool                      clangAssistedParsing;
     static QCString                  verifiedDotPath;
-    static volatile bool             terminating;
     static InputFileEncodingList     inputFileEncodingList;
     static std::mutex                countFlowKeywordsMutex;
     static std::mutex                addExampleMutex;

src/message.cpp

@@ -145,7 +145,6 @@ static void format_warn(const QCString &file,int line,const QCString &text)
   }
   if (g_warnBehavior == WARN_AS_ERROR_t::YES)
   {
-    Doxygen::terminating=true;
     exit(1);
   }
   g_warnStat = true;
@@ -160,7 +159,6 @@ static void handle_warn_as_error()
       QCString msgText = " (warning treated as error, aborting now)\n";
       fwrite(msgText.data(),1,msgText.length(),g_warnFile);
     }
-    Doxygen::terminating=true;
     exit(1);
   }
   g_warnStat = true;
@@ -285,7 +283,6 @@ void term_(const char *fmt, ...)
       fprintf(g_warnFile, "%s\n", "Exiting...");
     }
   }
-  Doxygen::terminating=true;
   exit(1);
 }
 
@@ -329,7 +326,6 @@ extern void finishWarnExit()
   if (g_warnStat && (g_warnBehavior == WARN_AS_ERROR_t::FAIL_ON_WARNINGS ||
                      g_warnBehavior == WARN_AS_ERROR_t::FAIL_ON_WARNINGS_PRINT))
   {
-    Doxygen::terminating=true;
     exit(1);
   }
 }