[lldb] Return StringRef from PluginInterface::GetPluginName

There is no reason why this function should be returning a ConstString.

While modifying these files, I also fixed several instances where
GetPluginName and GetPluginNameStatic were returning different strings.

I am not changing the return type of GetPluginNameStatic in this patch, as that
would necessitate additional changes, and this patch is big enough as it is.

Differential Revision: https://reviews.llvm.org/D111877

lldb/source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPython.cpp

@@ -590,10 +590,6 @@ ScriptInterpreterPythonImpl::~ScriptInterpreterPythonImpl() {
   PyGILState_Release(gil_state);
 }
 
-lldb_private::ConstString ScriptInterpreterPythonImpl::GetPluginName() {
-  return GetPluginNameStatic();
-}
-
 void ScriptInterpreterPythonImpl::IOHandlerActivated(IOHandler &io_handler,
                                                      bool interactive) {
   const char *instructions = nullptr;