`lamports_per_signature` in nonce account is not used

[tao-stones]

Problem

nonce account holds a fee_calculator that provides lamports_per_signature. However this lamports_per_signature is not used:

1. calculate_fee(...) uses fee_structure.lamports_per_siganture to calculate signature fee;
2. process_message(...) passes last block_hash's lamports_per_signature to transaction processing.

Proposed Solution

1. remove fee_calculator and lamports_per_signature() from nonce account
2. remove code (such as this) that fetches lamports_per_signature before calling calculate_fee(...)