remove all but truly stub support for SHARDING_FORK_{EPOCH,VERSION}

[tersec]

Currently, its only appearances in the consensus-specs repository are

consensus-specs$ rg SHARDING
specs/das/fork-choice.md
40:    if compute_epoch_at_slot(block.slot) < SHARDING_FORK_EPOCH:

tests/core/pyspec/eth2spec/test/context.py
14:    PHASE0, ALTAIR, BELLATRIX, CAPELLA, EIP4844, SHARDING,
311:    elif _check_current_version(spec, state, SHARDING):
315:        overrides['SHARDING_FORK_EPOCH'] = spec.GENESIS_EPOCH

tests/core/pyspec/eth2spec/test/sharding/unittests/test_get_start_shard.py
5:from eth2spec.test.helpers.constants import SHARDING
9:@with_phases([SHARDING])
26:@with_phases([SHARDING])
42:@with_phases([SHARDING])
60:@with_phases([SHARDING])
79:@with_phases([SHARDING])

tests/core/pyspec/eth2spec/test/altair/unittests/test_config_override.py
54:    assert spec.config.SHARDING_FORK_EPOCH == spec.GENESIS_EPOCH
55:    if state.fork.current_version == spec.config.SHARDING_FORK_VERSION:

tests/core/pyspec/eth2spec/test/phase0/unittests/fork_choice/test_on_attestation.py
27:    # elif spec.fork == SHARDING: TODO: check if vote count for shard blob increased as expected

tests/core/pyspec/eth2spec/test/phase0/sanity/test_blocks.py
576:# TODO All AttesterSlashing tests should be adopted for SHARDING and later but helper support is not yet there
821:    # if spec.fork == SHARDING:
898:# After SHARDING is enabled, a committee is computed for SHARD_COMMITTEE_PERIOD slots ago,
901:# TODO: when integrating SHARDING tests, voluntary-exit tests may need to change.

tests/core/pyspec/eth2spec/test/helpers/state.py
47:    Transition to slot `compute_epoch_at_slot(spec.config.SHARDING_FORK_EPOCH) + 1`
48:    and fork at `compute_epoch_at_slot(spec.config.SHARDING_FORK_EPOCH)`.
50:    transition_to(spec, state, spec.compute_epoch_at_slot(spec.config.SHARDING_FORK_EPOCH))

tests/core/pyspec/eth2spec/test/helpers/attestations.py
85:    # if spec.fork == SHARDING  # TODO: add extra data for shard voting
198:        # if spec.fork == SHARDING:  TODO: add shard data to attestation, include shard headers in block

tests/core/pyspec/eth2spec/test/helpers/constants.py
14:SHARDING = SpecForkName('sharding')

That is, it does not have any defined semantics, and it exists solely as a set of TODOs and boundary conditions.

https://github.com/eth-clients/eth2-networks and https://github.com/eth-clients/merge-testnets network definitions do to contain it, so this PR does not remove the spec/presets RuntimeConfig support for reading in that field from a such a network configuration. However, checkForkConsistency then asserts unconditionally that SHARDING_FORK_EPOCH is FAR_FUTURE_EPOCH and otherwise ignores it.

This PR was prompted by #4365 discovering that there's a spurious conflict between certain definitions of EIP4844_FORK_VERSION and SHARDING_FORK_VERSION, when the latter is irrelevant.

[github-actions]

Unit Test Results

         9 files  ±0       873 suites  ±0   24m 50s ⏱️ +20s
  2 704 tests ±0    2 511 ✔️ ±0  193 💤 ±0  0 ❌ ±0 
11 783 runs  ±0  11 564 ✔️ ±0  219 💤 ±0  0 ❌ ±0 

Results for commit d0b58cc. ± Comparison against base commit 4e71e77.

[etan-status]

RuntimeConfig parser has funtionality to ignore an irrelevant constant btw