SIANXKE-380: avoid file deletion before Attachment removal

[anx-abruckner]

This fix prevents the stored file from being deleted prematurely.
The previous version would delete the file even if a following validation (e.g. from another model within an admin form) would fail afterwards - which resulted in the attachment instance still existing in the db, but the referred file being gone (irrevocably).

[christophbuermann]

@anx-abruckner I don't understand the initial issue.

Using the following setup:

models.py

class TestModel(models.Model):
    name = models.CharField(max_length=50)
    attachments = AttachmentRelation()

admin.py

class TestAttachmentInlineAdmin(BaseAttachmentInlineAdmin):
    show_change_link = True


@admin.register(TestModel)
class TestModelAdmin(admin.ModelAdmin):
    list_display = ("name",)
    inlines = [
        TestAttachmentInlineAdmin,
    ]

If I cause a validation error in the admin change form by leaving the name empty and change the file of the attachment inside the inline, it falsely shows that the attachment is empty (before and after this pull request). After reloading the page (without sending POST data), the attachment is back in its original state and nothing is removed (again before and after this pull request).

The tests that have been modified by this pull request also run without errors, with or without the cleanup_file function that has been removed by this pull request.

[anx-abruckner]

@christophbuermann thank you for checking! The initial issue occurred in a project that copied the package's functionality - I assumed the core issue would also trouble the drf-attacments package, but I failed to reproduce the error. Hence I close this PR until further notice :P