mgr/dashboard: Refactoring of `DeletionModalComponent`

mgr/dashboard: Refactoring of `DeletionModalComponent`

- Simpler variable names:

    Examples:

	- `actionDescription` and `itemDescription` instead of `metaType`
	- `bodyTemplate` instead of `description`
	- `validationPattern` instead of `pattern`

    Some of these variable names have been generalized to ease the
    unification/generalization of dialog components:

	- `submitAction` instead of `deletionMethod`

- Removed unique `setUp` method.

    Benefits:

    - Creation of the component is done as intended by the developers of
    the `ngx-boostrap` package and as expected by developers which use
    the package. The `setUp` method does not have to be called anymore
    on the `DeletionModalComponent` exclusively but instead the
    component is instantiated as all other modals. Property assignment
    on the instantiated object isn't handled by the `setUp` method
    anymore but by the `modalService`.

    - With the removal of the `setUp` method, some tests could be
    removed as well.

    - No need to pass the reference of the created modal to the modal
    manually.

    Preserved:

    - The provided check within the `setUp` method, which checked if the
    component had been correctly instantiated, has been moved to the
    `ngOnInit` method of the component.

Signed-off-by: Patrick Nawracay <pnawracay@suse.com>