ranger: some code clean up

[winoros]

What problem does this PR solve?

The deleted code actually does not always take effect on the same range query.

mysql> create table t(a varchar(50), b int, index idx(a, b));
Query OK, 0 rows affected (0.01 sec)

mysql> explain select * from t where a > 'abc';
+-------------------+---------+------+-----------------------------------------------------------------------------------+
| id                | count   | task | operator info                                                                     |
+-------------------+---------+------+-----------------------------------------------------------------------------------+
| IndexReader_9     | 3333.33 | root | index:IndexScan_8                                                                 |
| └─IndexScan_8     | 3333.33 | cop  | table:t, index:a, b, range:("abc" +inf,+inf +inf], keep order:false, stats:pseudo |
+-------------------+---------+------+-----------------------------------------------------------------------------------+
2 rows in set (0.00 sec)

mysql> explain select a from t where a > 'abc';
+-------------------+---------+------+-------------------------------------------------------------------------+
| id                | count   | task | operator info                                                           |
+-------------------+---------+------+-------------------------------------------------------------------------+
| IndexReader_9     | 3333.33 | root | index:IndexScan_8                                                       |
| └─IndexScan_8     | 3333.33 | cop  | table:t, index:a, b, range:("abc",+inf], keep order:false, stats:pseudo |
+-------------------+---------+------+-------------------------------------------------------------------------+

What is changed and how it works?

Remove these codes won't make the result wrong. PrefixNext can promise correctness when we convert it to kvRange.

// PrefixNext returns the next prefix key.
//
// Assume there are keys like:
//
//   rowkey1
//   rowkey1_column1
//   rowkey1_column2
//   rowKey2
//
// If we seek 'rowkey1' Next, we will get 'rowkey1_column1'.
// If we seek 'rowkey1' PrefixNext, we will get 'rowkey2'.

This can help to simplify #8471 logic

Check List

Tests

• Unit test
• Integration test

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This change is 

[ngaut]

/run-all-tests

[alivxxx]

LGTM

[zz-jason]

LGTM