GS/GL/MTL: Misc fixes.

MTL: Add missing shader case for RGB5A1_TO_8I.
GL: Comment out vendor Nvidia.

Fixes warnings.

pcsx2/GS/Renderers/Metal/GSDeviceMTL.mm

@@ -1146,6 +1146,7 @@ bool GSDeviceMTL::Create(GSVSyncMode vsync_mode, bool allow_present_throttle)
 			case ShaderConvert::COPY:
 			case ShaderConvert::DOWNSAMPLE_COPY:
 			case ShaderConvert::RGBA_TO_8I: // Yes really
+			case ShaderConvert::RGB5A1_TO_8I:
 			case ShaderConvert::RTA_CORRECTION:
 			case ShaderConvert::RTA_DECORRECTION:
 			case ShaderConvert::TRANSPARENCY_FILTER:

pcsx2/GS/Renderers/OpenGL/GSDeviceOGL.cpp

@@ -605,7 +605,7 @@ bool GSDeviceOGL::CreateTextureFX()
 bool GSDeviceOGL::CheckFeatures(bool& buggy_pbo)
 {
 	//bool vendor_id_amd = false;
-	bool vendor_id_nvidia = false;
+	//bool vendor_id_nvidia = false;
 	//bool vendor_id_intel = false;
 
 	const char* vendor = (const char*)glGetString(GL_VENDOR);
@@ -618,7 +618,7 @@ bool GSDeviceOGL::CheckFeatures(bool& buggy_pbo)
 	else if (std::strstr(vendor, "NVIDIA Corporation"))
 	{
 		Console.WriteLn(Color_StrongGreen, "GL: NVIDIA GPU detected.");
-		vendor_id_nvidia = true;
+		//vendor_id_nvidia = true;
 	}
 	else if (std::strstr(vendor, "Intel"))
 	{