Remove the std feature implications from the lightning crate

Now that we don't have to have everything in our entire ecosystem
use the same `std`/`no-std` feature combinations we should start by
untangling our own features a bit.

This takes one last step, removing the implications of the `std`
feature from the `lightning` crate.

Author: TheBlueMatt

lightning/Cargo.toml

@@ -30,7 +30,7 @@ max_level_trace = []
 unsafe_revoked_tx_signing = []
 
 no-std = ["hashbrown", "possiblyrandom", "libm"]
-std = ["lightning-invoice/std", "bech32/std"]
+std = []
 
 # Generates low-r bitcoin signatures, which saves 1 byte in 50% of the cases
 grind_signatures = []