Remove completely unused 'database_ready' install state logic

So this was added back in #2352081: Prevent installation over an already installed database but it has never worked - database_ready was always FALSE then and nothing has changed since. Tests pass so this is obviously dead code and can be removed.

Confirmed it never gets set anywhere:

[mandelbrot:maintainer | Thu 23:45:36] $ grep -r "database_ready" *
core/includes/install.core.inc:    'database_ready' => FALSE,
core/includes/install.core.inc:  if ($install_state['database_verified'] && $install_state['database_ready']) {
core/includes/install.core.inc:    // install_verify_database_ready() will throw an AlreadyInstalledException
core/includes/install.core.inc:    install_verify_database_ready();
core/includes/install.core.inc:    'install_verify_database_ready' => [
core/includes/install.core.inc:      'run' => $install_state['database_ready'] ? INSTALL_TASK_SKIP : INSTALL_TASK_RUN_IF_NOT_COMPLETED,
core/includes/install.core.inc:function install_verify_database_ready() {

I wonder what this means for the install_verify_database_ready() task that #3120731: Incorrect "Drupal already installed" if any database settings are wrong or unsatisfactory also added a task to... maybe that's related to why that functionality went wrong in the first place?

Committed to 9.1.x, and cherry-picked to 9.0.x and 8.9.x. I didn't cherry-pick it to 8.8.x because the other installer change wasn't.

@longwave was incorrectly not credited in the commit message (sorry) due to crossposting with myself, but I've fixed this in the issue credits themselves.

Thanks!