看官方的说明
Each origin has an associated set of databases. Each
database has a name and a current version. There is no way to
enumerate or delete the databases available for an origin from this
API.
就是说,目前html5API中还没有删除现在数据库的API,所以不能删除整个数据库。变通的方法是可以使用SQL还删除数据库中的所有表。