refactor: remove unused imports in bip-0360 rust implementation

[strmfos]

сleaned up unused imports across the codebase:

• removed std::env from schnorr_example.rs and slh_dsa_key_gen.rs
• removed unused imports from slh_dsa_verification_example.rs (std::env, log::info, once_cell, bitcoin::hashes)
• removed Algorithm from data_structures.rs
• removed MixedLeafInfo from lib.rs
• removed tagged_hash from test files (p2mr_construction.rs, p2mr_pqc_construction.rs)

all test pass

[murchandamus]

I don’t think this adds substantial value.
@cryptoquick @EthanHeilman, please let me know if you want this to be reopened.