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.

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

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

In [18640]:

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

Good call on leaving validate_file().