Removing "page has moved" pointers

Marking WIP until I can test redirect behavior. Verified all of these changes on about.staging.gitlab.com

deleted:    source/handbook/account-management/account-expansion/index.html.md
deleted:    source/handbook/customer-success/engaging-solutions-architect/index.html.md
deleted:    source/handbook/customer-success/engaging/index.html.md
deleted:    source/handbook/customer-success/professional-services/index.html.md
deleted:    source/handbook/account-management/index.html.md
deleted:    source/handbook/account-management/premium_support_on-boarding/index.html.md

other pages behind this folder not sure how redirect will treat it and left in place for now.

/source/handbook/sales-process/index.html.md