Drop unused credentials.toml read/write from CLI v2 config

## Objective

The CLI v2 config command supports `config set access-key` and `config set secret-key`, which write credentials to `~/.backend.ai/credentials.toml`. However, this file is not consumed anywhere outside the v2 CLI itself, and the only realistic credential input for v2 is the `BACKEND_ACCESS_KEY`/`BACKEND_SECRET_KEY` environment variables. The credentials.toml support was introduced one month ago in PR #10504 (2026-03-25) as part of the initial v2 CLI scaffolding, so there is no legacy compatibility cost in removing it. Drop both the write path (config set access/secret) and the read path (load_v2_config reading credentials.toml).

## Acceptance Criteria

- `config set` no longer accepts `access-key` or `secret-key` keys.
- `load_v2_config()` no longer reads `~/.backend.ai/credentials.toml`.
- `config show` continues to display access/secret keys sourced from environment variables.

JIRA Issue: BA-5904