Remove the hard-coded German transliteration rules

I would propose to remove the hard-coded transliteration rules for German, because those rules do not apply to other countries using the same letters (in this case: Estonian).

This commit removes the hardcoded info and enables transliteration rules to be chosen with
`I18n.locale = :en`, like so:

```
# Select the correct locale
I18n.locale = :de

# First: Create the main object
sct = SEPA::CreditTransfer.new(
  # Name of the initiating party and debtor, in German: "Auftraggeber"
  # String, max. 70 char
  name: "Auftraggeber",
  ...
```