DM: Remove outdated roadmap.md

[alastori]

Description

The dm/roadmap.md file is outdated and no longer maintained. The roadmap references old issue links from the previous pingcap/dm repository and contains items that are no longer accurate.

Proposed Change

Remove the dm/roadmap.md file from the repository.

Rationale

• The roadmap is outdated and not being maintained
• Links reference the old pingcap/dm repository which has been archived
• Users with questions about DM roadmap items can reach out to the PingCAP team via Support or Community channels

[alastori]

cc @OliverS929