Remove unused 'dsn' function and its usage

Tests did not cover the 'dsn' function anyway.

Verified by calling

```
env OPENQA_BASEDIR=$PWD/t/data OPENQA_DATABASE=test script/openqa-gru
```

locally.

lib/OpenQA/Schema.pm

@@ -51,11 +51,6 @@ sub disconnect_db () {
     $SINGLETON = undef;
 }
 
-sub dsn {
-    my $self = shift;
-    return $self->storage->connect_info->[0]->{dsn};
-}
-
 sub deploy ($self, $force_overwrite = 0) {
     # lock config file to ensure only one thing will deploy/upgrade DB at once
     # we use a file in prjdir/db as the lock file as the install process and

lib/OpenQA/Shared/Plugin/Gru.pm

@@ -45,14 +45,8 @@ sub register ($self, $app, $config) {
     my $schema = $app->schema;
 
     my $conn = Mojo::Pg->new;
-    if (ref $schema->storage->connect_info->[0] eq 'HASH') {
-        $self->dsn($schema->dsn);
-        $conn->username($schema->storage->connect_info->[0]->{user});
-        $conn->password($schema->storage->connect_info->[0]->{password});
-    }
-    else {
-        $self->dsn($schema->storage->connect_info->[0]);
-    }
+    my $connect_info = $schema->storage->connect_info->[0];
+    $self->dsn($connect_info);
     $conn->dsn($self->dsn());
 
     # set the search path in accordance with the test setup done in OpenQA::Test::Database