chore(l1,l2): remove dead Environment fields difficulty and block_blob_gas_used

[Galoretka]

Removed unused fields difficulty and block_blob_gas_used from Environment in crates/vm/levm/src/environment.rs , and stopped populating them in:

• crates/vm/backends/levm/mod.rs::{setup_env, env_from_generic, generic_system_contract_levm}
• tooling/ef_tests/state/runner/levm_runner.rs::prepare_vm_for_tx
• tooling/ef_tests/state_v2/src/modules/runner.rs::get_vm_env_for_test

Reason: These fields are never read in opcode handlers or hooks. Post-merge execution uses prev_randao (EIP-4399) instead of difficulty. Blob fee logic relies on block_excess_blob_gas and per-tx fields; blob_gas_used is only relevant at block validation, not EVM runtime. This reduces cognitive load and avoids unnecessary environment initialization without changing execution semantics.