In branches.yaml, add poc/optional-proofs from developeruche/prysm and remove unused block-proposal-optimisation. (#318)

The `poc/optional-proofs` of the `developeruche/prysm` repository
contains a proof of concept of the [EL Optional
proofs](https://eips.ethereum.org/EIPS/eip-8025).

Let's use this repo/branch at the moment.

Signed-off-by: Manu NALEPA <enalepa@offchainlabs.com>

Author: nalepae

branches.yaml

@@ -98,10 +98,11 @@ prysm:
     - develop
     - master
     - bal-devnet-1
-    - block-proposal-optimisation
   alt_repos:
     MarcoPolo/prysm:
       - partial-columns
+    developeruche/prysm:
+      - poc/optional-proofs
 
 teku:
   branches: