feat: add remove_service_price to clear a configured service price

[Baskarayelu]

Summary

Adds remove_service_price(service_id) to the escrow contract, letting the admin
clear a previously configured per-request price and free its storage slot.

• Admin-gated (loads DataKey::Admin, then require_auth()); panics with
  NotInitialized (Enforce the documented 256-byte service-description cap in set_service_metadata #3) before init.
• Honours the pause gate: panics with ContractPaused (Emit structured events on admin, price, and registration state changes #4) when paused,
  consistent with other admin mutations (set_service_price,
  transfer_service_ownership, settle).
• Idempotent: removing the price of a never-priced service is a no-op.
• Removes DataKey::ServicePrice(service_id) and emits price_rm(service_id).
• No new error code (codes remain append-only, max Add a tiered pricing model with volume discounts per service #14).

Removed vs. set-to-zero

After removal, get_service_price and compute_billing read back 0, exactly
as for a never-priced service. The distinction is about storage, not the read
value: removal frees the storage slot and emits price_rm, whereas
set_service_price(service_id, 0) leaves a stored slot holding 0. Documented
in the /// docs and README.md.

Security notes

• State-changing entrypoint, so it is pause-gated and admin-authenticated like
  the sibling pricing/admin mutations.
• Read-only billing paths already default missing prices to 0, so freeing the
  slot cannot under/over-bill; usage counters are left untouched.
• Idempotent removal cannot panic on absent slots, avoiding a griefing surface.

Tests

set price then remove -> 0; idempotent no-op on never-priced; remove then
re-set; compute_billing is 0 after removal; non-admin caller is rejected;
price_rm event payload asserted via testutils Events; remove while paused
panics #4.

Closes #35

test result: ok. 58 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

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