gn: Remove an import() that's unused after https://codereview.chromium.org/2296953002

BUG=634184

Review-Url: https://codereview.chromium.org/2556843003
Cr-Original-Commit-Position: refs/heads/master@{#436702}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 3a2f4935b92906c059733e32fa0443dda98ab858

config/compiler/BUILD.gn

@@ -1545,7 +1545,6 @@ config("afdo") {
 # Full symbols.
 config("symbols") {
   if (is_win) {
-    import("//build/toolchain/goma.gni")
     cflags = [ "/Zi" ]  # Produce PDB file, no edit and continue.
 
     if (is_win_fastlink) {