[tint][utils] Remove stray '+'

Surprising that this isn't a compilation error.

Change-Id: I218669d96c0d5111cc02c8559e6e7fed01e738c6
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/161008
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
diff --git a/src/tint/utils/ice/ice.cc b/src/tint/utils/ice/ice.cc
index 9a4f981..f40d49e 100644
--- a/src/tint/utils/ice/ice.cc
+++ b/src/tint/utils/ice/ice.cc
@@ -54,7 +54,7 @@
 }
 
 std::string InternalCompilerError::Error() const {
-    return std::string(File()) + +":" + std::to_string(Line()) +
+    return std::string(File()) + ":" + std::to_string(Line()) +
            " internal compiler error: " + Message();
 }