Get rid of "default constants" in wp-settings

[nacin]

We have this block of code in wp-settings.php:

$default_constants = array( 'WP_POST_REVISIONS' => true );
foreach ( $default_constants as $c => $v )
        @define( $c, $v ); // will fail if the constant is already defined
unset($default_constants, $c, $v);

Since we haven't converted other constants in wp-settings.php over to this $default_constants convention, we should instead change this to the typical if !defined() define() we see throughout wp-settings.php.

At first, I was going to suggest we add a defined() check in that loop instead of suppressing errors, but removing it all together I think makes the most sense.

[nacin]

Improving performance is always good

[nacin]

Long gone, now default-constants.php, see #11881.