Drop explicit transaction from store_node_tokens_as_encrypted_value (#3280)

Migrations are executed in transactions anyway, and creating a savepoint can cause
spurious failures on databases that don't support transactional DDL (like
MySQL and MariaDB) when it attempts to commit a savepoint that was silently
not created because there wasn't an active transaction after some DDL was
executed.

While a better solution might involve splitting this migration into several so each
one is only DDL or only data manipulation, I don't think that can be done very
easily while maintaining compatibility with existing deployments.

Fixes #3229.

database/migrations/2020_04_10_141024_store_node_tokens_as_encrypted_value.php

@@ -33,7 +33,6 @@ class StoreNodeTokensAsEncryptedValue extends Migration
             $table->text('daemon_token')->change();
         });
 
-        DB::transaction(function () {
         /** @var \Illuminate\Contracts\Encryption\Encrypter $encrypter */
         $encrypter = Container::getInstance()->make(Encrypter::class);
 
@@ -45,7 +44,6 @@ class StoreNodeTokensAsEncryptedValue extends Migration
                 $datum->id,
             ]);
         }
-        });
 
         Schema::table('nodes', function (Blueprint $table) {
             $table->unique(['uuid']);