info_schema_tables: do not collect the (useless) sys schema

[frivoire]

This sys schema is another (mariadb-specific) "system" schema, introduced in MariaDB 10.6 (2021), cf: MariaDB's doc
EDIT: MySQL seems to be introducing also a sys schema in MySQL 8.4 (2024), cf: MySQL's doc

And it's not useful to collect metrics for this schema (and it's even counter-productive, cf the cost of storing the metrics afterwards).
So, let's just remove this schema, just like it's already performed for the other "system" schemas (ex: performance_schema) today to be consistent with all "system" schemas.

Note: it's my first PR here, so I'm not sure about the CI/tests, please forgive any mistake & tell me 🙏

[frivoire]

I'm estimating this to be a fairly simple PR, so (according to https://github.com/prometheus/mysqld_exporter/blob/main/CONTRIBUTING.md):
cc @SuperQ

Thanks :)