Fixes matomo-org#4990 Removing use of sessions during installation.

Removed installation step 'databaseCheck'
Factored out the System check in its own class
(The 3 strings keys were removed from otrance)

lang/en.json

@@ -1050,8 +1050,6 @@
         "Congratulations": "Congratulations",
         "CongratulationsHelp": "<p>Congratulations! Your Piwik installation is complete.<\/p><p>Make sure your JavaScript code is entered on your pages, and wait for your first visitors!<\/p>",
         "DatabaseAbilities": "Database abilities",
-        "DatabaseCheck": "Database Check",
-        "DatabaseClientVersion": "Database client version",
         "DatabaseCreation": "Database creation",
         "DatabaseErrorConnect": "Error while trying to connect to the database server",
         "DatabaseServerVersion": "Database server version",
@@ -1062,7 +1060,7 @@
         "DatabaseSetupServer": "Database Server",
         "DatabaseSetupTablePrefix": "Table Prefix",
         "Email": "email",
-        "ErrorInvalidState": "Error: it seems you tried to skip a step of the Installation process, or your cookies are disabled, or the Piwik configuration file was already created. %1$sMake sure your cookies are enabled%2$s and go back %3$s to the first page of the installation %4$s.",
+        "InvalidStateError": "Error: Piwik is already installed. %1$s Go back %3$s to Piwik%4$s.",
         "Extension": "extension",
         "Filesystem": "Filesystem",
         "GetInvolved": "If you like what you see, you can %1$sget involved%2$s.",