Debugger: Move struct sorting logic to generic parser

Backends only have to specify whether an object members are sortable
in principle (e.g. all structs), and some numeric 'sortgroup' value
for member items (higher values are always sorted on top).

Change-Id: I10ce94580374fed48a35f058a575a1408d6801af
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>

src/plugins/debugger/gdb/gdbengine.cpp

@@ -4596,7 +4596,6 @@ void GdbEngine::doUpdateLocals(const UpdateParameters &params)
 
     cmd.arg("resultvarname", m_resultVarName);
     cmd.arg("partialvar", params.partialVariable);
-    cmd.arg("sortstructs", boolSetting(SortStructMembers));
     cmd.callback = CB(handleFetchVariables);
     runCommand(cmd);