Drop constraint quota_definitions_name_key

[kathap]

This PR waits for issue #3949, if support for Mysql5.7 will be stopped.

On the name column in quota_defintions there were two unique constraints/indexes. One unique index with name qd_name_index, which is similar in postgres and mysql, and then an unique constraint on column name with name in postgres=quota_definitions_name_key and name in mysql=name. These index/constraint are created during table creation:

    create_table :quota_definitions do
      VCAP::Migration.common(self, :qd)

      String :name, null: false, unique: true, case_insensitive: true
      Boolean :non_basic_services_allowed, null: false
      Integer :total_services, null: false
      Integer :memory_limit, null: false

      index :name, unique: true
    end

The unique: true in line String :name, null: false, unique: true, case_insensitive: true creates the unique constraint.
Since there is an unique index created on column name index :name, unique: true (later renamed to qd_name_index), we don't need the unique constraint. Dropping it with this migration.

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