drm_hwcomposer: remove unused variable blend

The first tuple element returned by GetEnumValueWithName is just the index
of that enum, which is not useful.

Signed-off-by: Benjamin Li <benl@squareup.com>
diff --git a/drm/DrmPlane.cpp b/drm/DrmPlane.cpp
index 9dd44ff..5443e7e 100644
--- a/drm/DrmPlane.cpp
+++ b/drm/DrmPlane.cpp
@@ -209,19 +209,20 @@
     }
   } else {
     int ret = 0;
-    uint64_t blend = 0;
 
     switch (layer->blending) {
       case DrmHwcBlending::kPreMult:
-        std::tie(blend,
+        std::tie(std::ignore,
                  ret) = blend_property_.GetEnumValueWithName("Pre-multiplied");
         break;
       case DrmHwcBlending::kCoverage:
-        std::tie(blend, ret) = blend_property_.GetEnumValueWithName("Coverage");
+        std::tie(std::ignore,
+                 ret) = blend_property_.GetEnumValueWithName("Coverage");
         break;
       case DrmHwcBlending::kNone:
       default:
-        std::tie(blend, ret) = blend_property_.GetEnumValueWithName("None");
+        std::tie(std::ignore,
+                 ret) = blend_property_.GetEnumValueWithName("None");
         break;
     }
     if (ret) {