Clear parameter when editing a content pack

## Expected Behavior
When editing a content pack the user wishes to remove applied parameters from a entity.

## Current Behavior
When editing a content pack a applied parameter can't be removed from a entity since we do
not know what value to pass instead.

## Possible Solution
:man_shrugging:

## Steps to Reproduce (for bugs)
1. Create a content pack with a parameter applied to a entity
2. Create a new version of the content pack
3. Try to remove the parameter when creating the new version.

## Context
Since a content pack is shareable between user, a second user could try to
create a new version without installing the content pack. With that in mind we do
not have any value we could use to replace the parameter.

## Your Environment

* Graylog Version: 3.0.0