[lldb] Remove PluginInterface::GetPluginVersion

In all these years, we haven't found a use for this function (it has
zero callers). Lets just remove the boilerplate.

Differential Revision: https://reviews.llvm.org/D109600

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

@@ -595,8 +595,6 @@ lldb_private::ConstString ScriptInterpreterPythonImpl::GetPluginName() {
   return GetPluginNameStatic();
 }
 
-uint32_t ScriptInterpreterPythonImpl::GetPluginVersion() { return 1; }
-
 void ScriptInterpreterPythonImpl::IOHandlerActivated(IOHandler &io_handler,
                                                      bool interactive) {
   const char *instructions = nullptr;