Remove default :schema value

[mlarraz]

• This does not work on Rails 7 (See https://github.com/thuss/standalone-migrations/issues/169)
• This is actually redundant in all Rails versions supported by this library. If ENV['SCHEMA'] is not set, Rails has its own logic for determining the schema file name. By default, this will db/schema.rb.
• Keeps the ability to set the schema via file if not already set by env var.

[karloscarweber]

Why isn't this merged?

[thuss]

Thanks for this PR!