GSdx-Dialog: Remove useless member variable

The adapter ID was needed to be passed before for a legacy DX9 hack which is no longer present, I figured it could cause a potential conflict when the adapter value stored at INI is outdated. (which is now a possibility after
lightning removed reference device)

The other alternative would be to just force set the INI value on dialog initialization to avoid unavailable values in the INI file, but that would be a rough for a person transitioning from debug build to release build using the reference device
option, so just removing this outdated variable for now, I rather doubt we'd be needing it in the future, in case we need adapter passing to subdialogs, the former suggestion needs to be implemented. Not implementing it right now since there's no need for it.

plugins/GSdx/Window/GSSettingsDlg.cpp

@@ -612,7 +612,6 @@ bool GSShaderDlg::OnMessage(UINT message, WPARAM wParam, LPARAM lParam)
 
 GSHacksDlg::GSHacksDlg(const std::string &adapter_id)
 	: GSDialog{IDD_HACKS}
-	, m_adapter_id(adapter_id)
 	, m_old_skipdraw_offset{0}
 	, m_old_skipdraw{0}
 {

plugins/GSdx/Window/GSSettingsDlg.h

@@ -42,7 +42,6 @@ class GSShaderDlg : public GSDialog
 
 class GSHacksDlg : public GSDialog
 {
-	std::string m_adapter_id;
 	int m_old_skipdraw_offset;
 	int m_old_skipdraw;