externalWallHeatFluxTemperature: Removed "mode"

Heat power (Q), heat flux (q) and heat transfer coefficient (h)
can now all be specified simultaneously. Their effects will be summed
into a single heat transfer rate. The "mode" keyword is no longer
required.

tutorials/heatTransfer/chtMultiRegionFoam/reverseBurner/0/solid/T

@@ -25,7 +25,6 @@ boundaryField
     external
     {
         type            externalWallHeatFluxTemperature;
-        mode            coefficient;
         h               uniform 1e3;
         Ta              $internalField;
         value           $internalField;