Revert "Enhance Python font-lock to support multilines"

Revert "Enhance Python font-lock to support multilines"

This reverts commit 4915ca5dd4245a909c046e6691e8d4a1919890c8.

We have found that there are performance issues when editing a large
file.  The issue can be reproduced as follows:

1. emacs -Q
2. Open large Python file (e.g. turtle.py in Python)
3. Near the top of the buffer, enter open paren and some characters.

The above commit extends the region to be font-locked using
`python-nav-end-of-statement'.  However, if there are unbalanced
parens, it may move point to the end of the buffer.  This causes
almost the entire buffer to be font-locked, which is not acceptable
for large files.