[Doc][CARBONDATA-4331] Update the declaration of supported String data types

Why is this PR needed?
CHAR and VARCHAR as String data types are no longer supported in Carbon. They should be deleted from doc's desc.

What changes were proposed in this PR?
CHAR and VARCHAR stop appearing as two String data types in doc.

This closes apache#4258

Author: tangchuan

docs/supported-data-types-in-carbondata.md

@@ -36,8 +36,6 @@
 
   * String Types
     * STRING
-    * CHAR
-    * VARCHAR
 
     **NOTE**: For string longer than 32000 characters, use `LONG_STRING_COLUMNS` in table property.
     Please refer to TBLProperties in [CreateTable](./ddl-of-carbondata.md#create-table) for more information.