chore(l2): fix prover timing

[xqft]

Motivation

To measure execution and proving time correctly. This means removing the client initialization from the equation. In the case of ZisK, we use it's own timing exposed via a JSON file generated after proving.

[github-actions]

Lines of code report

Total lines added: 105
Total lines removed: 0
Total lines changed: 105

Detailed view

+----------------------------------------------+-------+------+
| File                                         | Lines | Diff |
+----------------------------------------------+-------+------+
| ethrex/crates/l2/prover/src/backend/exec.rs  | 58    | +16  |
+----------------------------------------------+-------+------+
| ethrex/crates/l2/prover/src/backend/risc0.rs | 130   | +25  |
+----------------------------------------------+-------+------+
| ethrex/crates/l2/prover/src/backend/sp1.rs   | 153   | +25  |
+----------------------------------------------+-------+------+
| ethrex/crates/l2/prover/src/lib.rs           | 97    | +39  |
+----------------------------------------------+-------+------+

[ilitteri]

Superseded by #5478