use __return_false|true instead of create_function

[TobiasBg]

The newly added functions __return_false and __return_true from #12381 should be used throughout core to replace those ugly create_function( ..., 'return true|false;' ); calls.

A quick grep only showed one occurance though, which can be changed with the attached patch.

I also suggest back-porting those two functions to BackPress.

[TobiasBg]

Patch replaces one occurance of create_function with new return_false

[nacin]

(Just so it doesn't get lost today in the feature freeze...)

[dd32]

(In [13546]) use return_false in preference to create_function(..return true..). Props TobiasBg. See #12435

[dd32]

Leaving this open for any other cases where this may be used that grep hasnt yet found

[westi]

I've done a sweep and can't find any so closing for now.