Revert of [gn] Switch custom snapsot bot to gn

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index e6384c6290dca291314163def329d108c65bd123..466fac63cce2be51c68e85965eb925167010be55 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -27,9 +27,6 @@
 
   # Enable compiler warnings when using V8_DEPRECATE_SOON apis.
   v8_imminent_deprecation_warnings = false
-
-  # Embeds the given script into the snapshot.
-  v8_embed_script = ""
 
   # Sets -dENABLE_DISASSEMBLER.
   v8_enable_disassembler = false
@@ -592,8 +589,6 @@
 
   script = "tools/run.py"
 
-  sources = []
-
   outputs = [
     "$target_gen_dir/snapshot.cc",
   ]
@@ -619,11 +614,6 @@
       "--startup_blob",
       rebase_path("$root_out_dir/snapshot_blob.bin", root_build_dir),
     ]
-  }
-
-  if (v8_embed_script != "") {
-    sources += [ v8_embed_script ]
-    args += [ rebase_path(v8_embed_script, root_build_dir) ]
   }
 }