Remove ifdef guards for ZIP_RDONLY

ZIP_RDONLY was introduced in libzip 1.0.0 which is required since
a979e9f

Author: tstarling

ext/zip/php_zip.c

@@ -1463,11 +1463,7 @@ PHP_METHOD(ZipArchive, open)
 	}
 
 	/* open for write without option to empty the archive */
-#ifdef ZIP_RDONLY
 	if ((flags & (ZIP_TRUNCATE | ZIP_RDONLY)) == 0) {
-#else
-	if ((flags & ZIP_TRUNCATE) == 0) {
-#endif
 		zend_stat_t st = {0};
 
 		/* exists and is empty */

ext/zip/php_zip.stub.php

@@ -82,12 +82,10 @@ class ZipArchive implements Countable
      * @cvalue ZIP_OVERWRITE
      */
     public const int OVERWRITE = UNKNOWN;
-#ifdef ZIP_RDONLY
     /**
      * @cvalue ZIP_RDONLY
      */
     public const int RDONLY = UNKNOWN;
-#endif
 
     /**
      * @cvalue ZIP_FL_NOCASE