remove `persistent_storage` from docs (#385)

docs/en/manuals/html5.md

@@ -201,9 +201,6 @@ Object `extra_params` is an optional object that can have the following fields:
 'engine_arguments':
     List of arguments (strings) that will be passed to the engine.
 
-'persistent_storage':
-    Boolean toggling the usage of persistent storage.
-
 'custom_heap_size':
     Number of bytes specifying the memory heap size.
 

docs/zh/manuals/html5.md

@@ -201,9 +201,6 @@ DEFOLD_ENGINE_ARGUMENTS
 'engine_arguments':
     传入引擎的参数列表 (字符串).
 
-'persistent_storage':
-    是否使用持久化存储的布尔值.
-
 'custom_heap_size':
     自定义内存使用的大小.