refactor(telemetry): pull consensus-tracing scope-leak out of phase-3

Commit c6c019ed8b ("addressed code review comments") bundled tx-tracing
review fixes with consensus-tracing scaffolding that belongs on
pratik/otel-phase4-consensus-tracing (PR #6426). This commit lifts the
consensus-only parts back out of phase-3 so PR #6425 stays scoped to
transaction tracing. Phase-4 already carries the same content (via prior
phase-3 -> phase-4 merges) plus its own evolution on top, so nothing is
moved across — only removed here.

Removed:
  - src/xrpld/app/consensus/ConsensusSpanNames.h (deleted; phase-4 owns
    it).
  - PeerImp.cpp: revert onMessage(TMProposeSet)/onMessage(TMValidation)
    consensus-attr setAttribute calls to the hardcoded
    "xrpl.consensus.{trusted,round,ledger.seq}" strings used before
    c6c019ed8b. Drop the now-unused
    #include <xrpld/app/consensus/ConsensusSpanNames.h>.
  - RCLConsensus::Adaptor::validate(): remove the
    TODO(observability/secure-OTel) block on validation trace_context.
  - TraceContextPropagator.h: remove the TODO(observability/secure-OTel)
    block on injectToProtobuf().

Tx-tracing parts of c6c019ed8b are intentionally untouched.

No phase-3 caller of telemetry::consensus_span:: remains; verified via
git grep. No test on phase-3 references the removed header.

Signed-off-by: Pratik Mankawde <3397372+pratikmankawde@users.noreply.github.com>

include/xrpl/telemetry/TraceContextPropagator.h

@@ -91,14 +91,6 @@ injectToProtobuf(opentelemetry::context::Context const& ctx, protocol::TraceCont
 
     // Serialize flags
     proto.set_trace_flags(spanCtx.trace_flags().flags());
-
-    // TODO(observability/secure-OTel): the protobuf TraceContext message
-    // also carries `trace_state` (field 4), which is currently neither
-    // populated here nor read by extractFromProtobuf above. The field is
-    // reserved for the secure tracing pipeline outlined in
-    // OpenTelemetryPlan/secure-OTel.md, where an authenticated token in
-    // tracestate will let receivers reject spoofed/poisoned trace context.
-    // Wire trace_state through inject/extract once the consumer lands.
 }
 
 }  // namespace telemetry