[SQL][Minor] Remove cache keyword in SqlParser

Since cache keyword already defined in `SparkSQLParser` and `SqlParser` of catalyst is a more general parser which should not cover keywords related to underlying compute engine, to remove  cache keyword in  `SqlParser`.

Author: wangfei <wangfei1@huawei.com>

Closes #4393 from scwf/remove-cache-keyword and squashes the following commits:

10ade16 [wangfei] remove cache keyword in sql parser

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/SqlParser.scala

@@ -57,7 +57,6 @@ class SqlParser extends AbstractSparkSQLParser {
   protected val AVG = Keyword("AVG")
   protected val BETWEEN = Keyword("BETWEEN")
   protected val BY = Keyword("BY")
-  protected val CACHE = Keyword("CACHE")
   protected val CASE = Keyword("CASE")
   protected val CAST = Keyword("CAST")
   protected val COALESCE = Keyword("COALESCE")