Remove cflags for asm-only target

Since they are no longer passed as arguments to the assemblers.
BUG=none
R=andrew@webrtc.org

Review URL: https://codereview.webrtc.org/1378503002 .

Patch from Andrew Bonventre <andybons@chromium.org>.
diff --git a/dl/BUILD.gn b/dl/BUILD.gn
index 2e0e368..7fa5dae 100644
--- a/dl/BUILD.gn
+++ b/dl/BUILD.gn
@@ -199,9 +199,6 @@
 
     if (!arm_use_neon) {
       configs -= [ "//build/config/compiler:compiler_arm_fpu" ]
-      # TODO(andybons): Remove cflags once the toolchains have been updated
-      # to use asmflags for assemblers.
-      cflags = [ "-mfpu=neon" ]
       asmflags = [ "-mfpu=neon" ]
     }