Cycles SocketAlgo : Fix warning format arguments

There was one too many, which caused `boost::format()` to throw, subsequently crashing Gaffer.

I broke this in 8454f37, where I confidently stated that we didn't need to support transform sockets. But the `texture_coordinate` shader does have such a socket, called `ob_tfm`, which is how this problem surfaced. But I'm still not convinced we need to support it; I'm not sure how a user would use `ob_tfm` - maybe it's more of an implementation detail for Blender?

Fixes GafferHQ#5147.

python/GafferCyclesTest/IECoreCyclesPreviewTest/RendererTest.py

@@ -1818,8 +1818,8 @@ def testUnsupportedShaderParameters( self ) :
 		self.assertEqual( len( mh.messages ), 1 )
 		self.assertEqual( mh.messages[0].context, "Cycles::SocketAlgo" )
 		self.assertEqual( mh.messages[0].level, IECore.Msg.Level.Warning )
-		six.assertRegex(
-			self, mh.messages[0].message,
+		self.assertRegex(
+			mh.messages[0].message,
 			"Unsupported socket type `transform` for socket `ob_tfm` on node .*"
 		)
 

src/GafferCycles/IECoreCyclesPreview/SocketAlgo.cpp

@@ -46,6 +46,8 @@ IECORE_PUSH_DEFAULT_VISIBILITY
 #include "util/vector.h"
 IECORE_POP_DEFAULT_VISIBILITY
 
+#include "fmt/format.h"
+
 using namespace std;
 using namespace Imath;
 using namespace IECore;
@@ -448,8 +450,10 @@ void setSocket( ccl::Node *node, const ccl::SocketType *socket, const IECore::Da
 		default:
 			IECore::msg(
 				IECore::Msg::Warning, "Cycles::SocketAlgo",
-				boost::format( "Unsupported socket type `%1%` for socket `%2%` on node `%3%`." )
-					% ccl::SocketType::type_name( socket->type ) % socket->name % node->name
+				fmt::format(
+					"Unsupported socket type `{}` for socket `{}` on node `{}`.",
+					ccl::SocketType::type_name( socket->type ), socket->name, node->name
+				)
 			);
 			break;
 	}