Libevent log category was partially removed in 39e9099da5, and this
commit extends that with the following changes:
- Stops showing libevent in the list of supported log categories in
`bitcoind -help` and `bitcoin-cli help logging` output.
- Stops returning `"libevent": false` in `logging` RPC output.
It's not good to treat libevent as a supported log category when it
can't be enabled and trying to enable it results in warnings.
There's also no need to define an unused LIBEVENT constant value and
keep more complicated logic for dealing with deprecated log categories,
so this change also simplifies code internally.
Co-authored-by: David Gumberg <davidzgumberg@gmail.com>
Co-authored-by: l0rinc <pap.lorinc@gmail.com>
