remove unused import

Author: Bellangelo

src/Rules/Keywords/RequireFileExistsRule.php

@@ -10,7 +10,6 @@
 use PHPStan\Rules\RuleErrorBuilder;
 use PHPStan\ShouldNotHappenException;
 use function is_file;
-use function is_string;
 use function sprintf;
 use function str_replace;