[aspnet5] REMOVED

[jimschubert]

PR checklist

• Read the contribution guidelines.
• Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates)
• Filed the PR against the correct branch: master for non-breaking changes and 2.3.0 branch for breaking (non-backward compatible) changes.

Description of the PR

Removes the deprecated aspnet5 generator.

This is the last outstanding item for #3085

Three locations in the wiki also need to be updated:

$ grep -Rn 'aspnet5' .
./Server-stub-generator-HOWTO.md:330:  -o samples/server/petstore/aspnet5
./Server-stub-generator-HOWTO.md:333:`aspnet5` has been deprecated. Please use `aspnetcore` instead.
./Server-stub-generator-HOWTO.md:365:aspnet5 (deprecated and replaced by aspnetcore)