[mksnapshot] Remove target OS defines

These were added to support specific cross-compile scenarios that
should now be covered by the --target-os and --target-arch flags.

Bug: v8:9103
Change-Id: I848f4f39cecc83f9ec8529c4f8855662059e3dc5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1624219
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61787}

BUILD.gn

@@ -3816,14 +3816,6 @@ if (v8_use_snapshot && current_toolchain == v8_snapshot_toolchain) {
       ":v8_nosnapshot",
       "//build/win:default_exe_manifest",
     ]
-
-    if (target_os == "fuchsia") {
-      defines = [ "V8_TARGET_OS_FUCHSIA" ]
-    } else if (target_os == "win") {
-      defines = [ "V8_TARGET_OS_WIN" ]
-    } else if (target_os == "chromeos") {
-      defines = [ "V8_TARGET_OS_CHROMEOS" ]
-    }
   }
 }