CodeAnalysisRuleSet property is being cleared when deleting an unused ruleset file from a project

**Visual Studio Version**: 2015, 2019

**Summary**:
When you delete a ruleset file from a project, the CodeAnalysisRuleSet property is being cleared even when it was not referencing the deleted ruleset.
Happens in both old-style and new-style projects.

**Steps to Reproduce**:

1. Create a project with rulesets A and B.

2. Set CodeAnalysisRuleSet to ruleset A.

3. Delete ruleset B.

**Expected Behavior**: CodeAnalysisRuleSet still points to ruleset A.

**Actual Behavior**: CodeAnalysisRuleSet is cleared.

**User Impact**: User's rulesets are being reset without the user knowing about it.

[CodeAnalysisRuleSetBeingCleared.zip](https://github.com/dotnet/project-system/files/4734788/CodeAnalysisRuleSetBeingCleared.zip)