MDEV-33927 Wrong error message while altering the table with foreign key relationship

[Thirunarayanan]

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• [x] The Jira issue number for this PR is: MDEV-33927

Description

• Alter statement using copy algorithm throws wrong message "Cannot delete rows from table which is parent in a foreign key constraint" even though we expect duplicate key for the index.

copy_data_between_tables(): Remove the error condition to throw ER_FK_CANNOT_DELETE_PARENT and make sure that ALTER IGNORE shouldn't allow to break unique of foreign key constraints.

How can this PR be tested?

./mtr innodb.fk_col_alter

Basing the PR against the correct MariaDB version

• [ ] This is a new feature or a refactoring, and the PR is based against the latest MariaDB development branch.
• [x] This is a bug fix, and the PR is based against the earliest maintained branch in which the bug can be reproduced.

PR quality check

• [x] I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• [x] For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.

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