chore: remove .agents/skills/deepgram-dotnet-maintaining-sdk (moved to deepgram/agents) [no-ci] (#403)

The per-language SDK maintenance skill duplicated AGENTS.md content
(its own footer admitted this) and was internal-facing -- targeted
at the SDK team running regen cycles, not at SDK users.

The maintenance workflow has been generalized and consolidated into
the internal deepgram/agents repo (PR #16): deepgram-sdk-maintenance,
deepgram-sdk-prepare-regen, deepgram-sdk-review-regen. AGENTS.md in
this repo remains as the SDK-specific source of truth for frozen
file lists and validation commands.

Removing the duplicate makes `deepgram/agents` + AGENTS.md the
canonical pair (workflow + per-repo specifics). No information is
lost -- AGENTS.md has the full per-repo detail.

See: https://github.com/deepgram/agents/pull/16

Author: lukeocodes