Assigned issue count not updated correctly on project delete
### Summary
When a project is deleted the assigned issue count is not updated correctly
### Steps to reproduce
1. Clear all your todos
1. Create a project
1. Create an issue
1. Assign the issue to yourself - note you now have a todo
1. Delete the project - note you still have a todo count but no todos
### What is the current *bug* behavior?
Assigned issue count shows as 1 or greater when no assigned issues exist
### What is the expected *correct* behavior?
Assigned issue count shows as 0 when no assigned issues exist
### Relevant logs and/or screenshots

### Output of checks
This bug happens on GitLab.com
#### Results of GitLab environment info
This bug happens on GitLab.com
#### Results of GitLab application Check
This bug happens on GitLab.com
### Possible fixes
Related to previous count issue with todos: https://gitlab.com/gitlab-org/gitlab-ce/issues/23919
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