Remove function definition order rule

The Google c++ style guide removed this rule in 2014. There was support on cxx
for removing this rule from the Chromium style guide.

Change-Id: I6434b2c2112638cfe96e8c42b59e20889d19aa9f
Reviewed-on: https://chromium-review.googlesource.com/c/1330935Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Dan H <harringtond@google.com>
Cr-Commit-Position: refs/heads/master@{#607625}

styleguide/c++/c++.md

@@ -54,8 +54,6 @@ separate [C++11 use in Chromium](https://chromium-cpp.appspot.com/) page.
 
   * Prefer `(foo == 0)` to `(0 == foo)`.
 
-  * Function declaration order should match function definition order.
-
   * Prefer putting delegate classes in their own header files. Implementors of
     the delegate interface will often be included elsewhere, which will often
     cause more coupling with the header of the main class.