VideoCommon: Get rid of the global g_available_video_backends

Replace it with a function-local static that is initialized on first
use. This gets rid of a global variable and removes the need for
manual initialization in UICommon.

This commit also replaces the weird find_if that looks for a non-null
unique_ptr with a simple "is vector empty" check considering that
none of the pointers can be null by construction.

Source/Core/UICommon/UICommon.cpp

@@ -97,7 +97,6 @@ void Init()
   SConfig::Init();
   Discord::Init();
   Common::Log::LogManager::Init();
-  VideoBackendBase::PopulateList();
   WiimoteReal::LoadSettings();
   GCAdapter::Init();
   VideoBackendBase::ActivateBackend(Config::Get(Config::MAIN_GFX_BACKEND));
@@ -109,7 +108,6 @@ void Shutdown()
 {
   GCAdapter::Shutdown();
   WiimoteReal::Shutdown();
-  VideoBackendBase::ClearList();
   Common::Log::LogManager::Shutdown();
   Discord::Shutdown();
   SConfig::Shutdown();