MemoryCache - Remove unhandled exception handler.

[StephenMolloy]

The unhandled exception handler only disposes the stats timer in .Net Core, and doing that dispose in an unhandled exception could result in a deadlocked process. The unhandled exception handler is not necessary anymore without the perf counters that existed in NetFx.

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.