Remove legacy welcome page code

[JacksonKearl]

This iteration I'll be removing code for the old style welcome page. It is currently available via the hidden legacy_welcomePage setting of workbench.startupEditor, but based on no feedback seeking options for enabling it I think it is safe to remove entirely.

[JacksonKearl]

Assigning bug label for verification. Things to verify: if you open some welcome pages in a version that supports legacy_welcomePage, then upgrade to a version that does not support, is the failover process smooth?