School country code to be redundant

[puqeko]

Following #125 and as mentioned in #128. All schools now in the database should be NZ schools. However, statements such as School.where(country_code: "NZ") will not work if this field is set to nil or removed from the database.

Suggested tasks:

• Remove country code field from registration form.
• Remove country code from database and create migration.
• Ensure that all mentions of School.where(country_code: "NZ") are replaced across the site.