Drop unsupported MySQL driver

[jonasraoni]

Describe the bug
The support for the old MySQL driver has been dropped in PHP 7.0, so we can do the same, at least for the main branch.

PRs

• pkp/pkp-lib#8099 Dropped references to the deprecated MySQL driver #8138
• Feature/main/8099 drop deprecated mysql driver ops#321
• Feature/main/8099 drop deprecated mysql driver ojs#3496
• Feature/main/8099 drop deprecated mysql driver omp#1176

What application are you using?
*/main