build: remove the vendor directory (#28883)

The `vendor` directory is a remnant of times long since gone when we had
brightray and libchromiumcontent (never forget).  It is currently the
hope to a single header file that afaics we do not need anymore and
randomly pyyaml gets cloned there but that is easily movable.

This commit removes all references to the vendor directory from scripts,
docs, helpers, etc. and removes the directory itself.

script/lib/native_tests.py

@@ -4,10 +4,9 @@ import os
 import subprocess
 import sys
 
-SOURCE_ROOT = os.path.abspath(
-    os.path.dirname(os.path.dirname(os.path.dirname(__file__))))
-VENDOR_DIR = os.path.join(SOURCE_ROOT, 'vendor')
-PYYAML_LIB_DIR = os.path.join(VENDOR_DIR, 'pyyaml', 'lib')
+from util import SRC_DIR
+
+PYYAML_LIB_DIR = os.path.join(SRC_DIR, 'third_party', 'pyyaml', 'lib')
 sys.path.append(PYYAML_LIB_DIR)
 import yaml  #pylint: disable=wrong-import-position,wrong-import-order