Drop support for CHANGE MASTER savefiles from MySQL/MariaDB 5.5 and before

Starting with MySQL/MariaDB 4.1 for @master_info_file and
5.6/10.0 for @relay_log_info_file, the files switched to a slightly different but more extensible format, where the first line of the file indicates the number of valid lines in the file.

MariaDB 5.5 has been EOL for 6 years, and MySQL 5.5 for almost 7.5 years.
Therefore, it is no longer a priority to have a portion of code slightly divergent to support upgrades from those versions.