BLS activate number should use block number otherwise height

[Pana]

https://github.com/Conflux-Chain/conflux-rust/blob/master/crates/execution/executor/src/machine/mod.rs#L242

The activation height of BLS precompiled contracts should use block number like other precompiles, as the block number is uniformly used by the Machine.builtin method to determine whether the precompile is activated.

However, the current implementation code has no real side effects, so it can be kept; this issue is only used for record-keeping.