docs(l2): remove proposer in README.md (#2470)

**Motivation**

The `proposer.md` file was renamed to `sequencer.md`, and the old link
in the docs index became obsolete.

**Description**

Hi! I removed the outdated reference to `proposer.md` in
`crates/l2/docs/README.md` since it's now covered under `sequencer.md`.
ref:
https://github.com/lambdaclass/ethrex/pull/2269/files#diff-95ad85cd4c72b932973f93785a8a1f365b56757d2972fe671ff33221b7bd0546

Co-authored-by: Javier Rodríguez Chatruc <49622509+jrchatruc@users.noreply.github.com>

Author: cypherpepe

crates/l2/docs/README.md

@@ -9,7 +9,6 @@ For more detailed documentation on each part of the system:
 - [Sequencer](./sequencer.md)
 - [Contracts](./contracts.md)
 - [Execution program](./program.md)
-- [Proposer](./proposer.md)
 - [Prover](./prover.md)
 - [State Diffs](./state_diffs.md)
 - [Withdrawals](./withdrawals.md)