Remove SQLite module from Performance Lab plugin

[felixarntz]

Follow-up to #737: The SQLite module should be removed from the Performance Lab plugin after the upcoming Performance Lab 2.4 release (June 19), i.e. as part of the following 2.5 release (July 17).

Note that this effort is decoupled from #656, since the SQLite module is not being removed as part of that effort, but rather simply due to the above decision. So it shouldn't need to be blocked by the efforts in the greater unbundling effort.

Requirements

• Remove database/sqlite module entirely, as well as any references to it in the files phpcs.xml.dist, admin/load.php.
• Remove the migration code that was added as part of Implement notice to prompt administrator to update to SQLite plugin instead of SQLite module #737.
• Add an upgrade notice informing about the removal of the module to the end of readme.txt to accompany the 2.5 release:

== Upgrade Notice ==

= 2.5.0 =

The SQLite module is no longer present starting with this release. If you still use it, please migrate to the standalone plugin before updating.