Do not overwrite loguru global stderr verbosity

- This fixes a bug when applications completely rely on
  vtkLogger::Init(ac, av) to parse the -v flag on CLI and
  set the stderr verbosity.
- With this commit, it is finally possible to run the test
  executables with a custom verbosity by merely passing -v <verbosity>
  on the command line. This greatly simplifies debugging trace messages
  without having to manually set the stderr verbosity and recompile the
  test executable.

Common/Core/vtkLogger.cxx

@@ -135,7 +135,6 @@ void vtkLogger::Init(int& argc, char* argv[], const char* verbosity_flag /*= "-v
   loguru::g_preamble_time = false;
   loguru::g_internal_verbosity = static_cast<loguru::Verbosity>(vtkLogger::InternalVerbosityLevel);
 
-  const auto current_stderr_verbosity = loguru::g_stderr_verbosity;
   if (loguru::g_internal_verbosity > loguru::g_stderr_verbosity)
   {
     // this avoids printing the preamble-header on stderr except for cases
@@ -156,8 +155,8 @@ void vtkLogger::Init(int& argc, char* argv[], const char* verbosity_flag /*= "-v
   {
     options.main_thread_name = detail::ThreadName;
   }
-  loguru::init(argc, argv, options);
-  loguru::g_stderr_verbosity = current_stderr_verbosity;
+  loguru::init(
+    argc, argv, options); // initializes many things, one of them being g_stderr_verbosity
 #else
   (void)argc;
   (void)argv;