[clang-tidy] Remove PRESUBMIT check for NULL

This change removes the PRESUBMIT check for NULL, since it is rendered
obsolete by the modernize-use-nullptr clang-tidy checked introduced in
r709137. The latter check produces less false positives, since it only
runs on changed lines, rather than across changed files.

Bug: None
Change-Id: I0ee29c95e1e53a6f421377932a85edff1466b6b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2584162
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836027}

PRESUBMIT.py

@@ -399,14 +399,6 @@ _NOT_CONVERTED_TO_MODERN_BIND_AND_CALLBACK = '|'.join((
 # * Error flag. True if a match is a presubmit error, otherwise it's a warning.
 # * Sequence of paths to *not* check (regexps).
 _BANNED_CPP_FUNCTIONS = (
-    (
-      r'/\bNULL\b',
-      (
-       'New code should not use NULL. Use nullptr instead.',
-      ),
-      False,
-      (),
-    ),
     (
       r'/\busing namespace ',
       (