Delete the v8_target_cpu/v8_current_cpu hack in the GN build.

This finishes the work in bug 625353 to make
v8_target_cpu/v8_current_cpu work correctly across multiple
toolchains, by deleting the workaround that was needed to
get the changes to land across repos.

TBR=machenbach@chromium.org
BUG=625353

Review-Url: https://codereview.chromium.org/2157733002
Cr-Commit-Position: refs/heads/master@{#405933}

build/toolchain/gcc_toolchain.gni

@@ -475,12 +475,6 @@ template("gcc_toolchain") {
         v8_current_cpu = current_cpu
       }
 
-      # TODO(crbug.com/625353) - Delete after v8 has been updated
-      # to only refer to v8_current_cpu. Until then, we need to make
-      # sure that v8_current_cpu always has the same value as v8_target_cpu,
-      # so that //build defines evaluate the way v8 is expecting them to.
-      v8_current_cpu = v8_target_cpu
-
       # Disable sanitizers for non-default toolchains.
       is_asan = false
       is_cfi = false