minor #6363 Removed info about reducing visibility for private (AAstakhov)

This PR was merged into the 2.3 branch.

Discussion
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Removed info about reducing visibility for private

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | n/a

It makes no sense to reduce visibility of private methods as soon as it is minimal visibility.
I removed this table line in order to not confuse readers.

Commits
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bc6a898 Removed info about reducing visibility for private

Author: weaverryan

contributing/code/bc.rst

@@ -236,7 +236,6 @@ Change return type                                  No
 Add private method                                  Yes
 Remove private method                               Yes
 Change name                                         Yes
-Reduce visibility                                   Yes
 Add argument without a default value                Yes
 Add argument with a default value                   Yes
 Remove argument                                     Yes