Prune dead code from when importers were in core.

[westi]

In wp-admin/import we have:

// Load all importers so that they can register.
$import_loc = 'wp-admin/import';
$import_root = ABSPATH.$import_loc;
$imports_dir = @ opendir($import_root);
if ($imports_dir) {
	while (($file = readdir($imports_dir)) !== false) {
		if ($file[0] == '.') {
			continue;
		} elseif (substr($file, -4) == '.php') {
			require_once($import_root . '/' . $file);
		}
	}
}
@closedir($imports_dir);

But we don't have that folder any more.

We should rip the code out so we don't have unnecessary suppressed notices.

[westi]

Also need to take code out of admin.php too.

[duck_]

The validate_file call in admin.php doesn't look necessary now, so we might like to remove it as well.

[duck_]

In [18640]:

Pull out old import remnant from when importers were shipped in core. Fixes #15844.

[nacin]

Good call on leaving validate_file().