Remove /d2Zi+ build flag

/d2Zi+ is replaced by /Zo and /Zo is enabled by default.
https://docs.microsoft.com/en-us/cpp/build/reference/zo-enhance-optimized-debugging

clang-cl ignores this option.
https://github.com/llvm-project/llvm-project-20170507/blob/fdd1e44ea4b98dd1c02d5f2181cca6bbb854aadd/clang/include/clang/Driver/CLCompatOptions.td#L319

Change-Id: I35d24b6d3c5d7e3c3dbd2e0c37f41683b062f650
Reviewed-on: https://chromium-review.googlesource.com/890572Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@google.com>
Cr-Commit-Position: refs/heads/master@{#532364}

build/config/compiler/BUILD.gn

@@ -1598,7 +1598,6 @@ if (is_win) {
   common_optimize_on_cflags = [
     "/Ob2",  # Both explicit and auto inlining.
     "/Oy-",  # Disable omitting frame pointers, must be after /O2.
-    "/d2Zi+",  # Improve debugging of optimized code.
     "/Zc:inline",  # Remove unreferenced COMDAT (faster links).
   ]
   if (!is_asan) {