refactor(cli): clean up profile selector logic and remove legacy config

### What happened?
There is lingering technical debt in the CLI configuration regarding the legacy profileSelector logic:

See [config.ts](https://github.com/google-gemini/gemini-cli/blob/main/packages/cli/src/config/config.ts#L941-L950)

` ypescript
// TODO(joshualitt): Clean this up alongside removal of the legacy config.
let profileSelector: string | undefined = undefined;
`

### What did you expect to happen?
This legacy config mapping should be completely removed and cleaned up to streamline how Config initializes and reduces unnecessary branching logic.

### Client information
N/A - Tech Debt Cleanup

Start in packages/cli/src/config/config.ts around lines 941-950, then search for the remaining profileSelector and legacy-config references. Done means the legacy profile config mapping and its unnecessary initialization branching are removed from Config, leaving the CLI configuration logic clean.

Written by the indexing model from the issue text.