[SPARK-2734][SQL] Remove tables from cache when DROP TABLE is run.

Seems we also need to support refer to a table with the format of `dbName.tableName` and `IF EXISTS`.

An example AST:
The AST tree of `drop table if exists default.src` is 
```
TOK_DROPTABLE
  TOK_TABNAME
    default
    src
  TOK_IFEXISTS
```