Delete all custom fields at once?

You can use the SQL way. Go in your database tool, like phpMyAdmin or Adminer (also possible via Plugin in the WordPress back-end). Use the SQL area for creating custom queries.

Before you start, create a backup of the database and also a single backup for the table _postmeta.

Change the string wpbeta_2 to your prefix, this is only a example.

Get all fields that have a custom field:

SELECT *  FROM `wpbeta_2_postmeta` WHERE `meta_key` != ''

Now you have a list of all fields, check this.

Delete a specific field:

DELETE FROM `wpbeta_2_postmeta` WHERE `meta_key` = 'modules'

Delete all fields:

DELETE FROM `wpbeta_2_postmeta` WHERE `meta_key` != ''