idl_parser: Always use PLY from third_party.

tools/idl_parser/idl_lexer.py

Index: tools/idl_parser/idl_lexer.py
diff --git a/tools/idl_parser/idl_lexer.py b/tools/idl_parser/idl_lexer.py
index 1f186d722d9e5921061d1862b604b3cfef4da907..a5fc49c3e251aa91de2678721342168e95f5235b 100755
--- a/tools/idl_parser/idl_lexer.py
+++ b/tools/idl_parser/idl_lexer.py
@@ -17,20 +17,10 @@ PLY can be found at:
 import os.path
 import sys
 
-#
-# Try to load the ply module, if not, then assume it is in the third_party
-# directory.
-#
-try:
-  # Disable lint check which fails to find the ply module.
-  # pylint: disable=F0401
-  from ply import lex
-except ImportError:
-  module_path, module_name = os.path.split(__file__)
-  third_party = os.path.join(module_path, '..', '..', 'third_party')
-  sys.path.append(third_party)
-  # pylint: disable=F0401
-  from ply import lex
+SRC_DIR = os.path.join(os.path.dirname(__file__), os.pardir, os.pardir)
+sys.path.insert(0, os.path.join(SRC_DIR, 'third_party'))
+from ply import lex
+
 
 #
 # IDL Lexer