Remove Pyrmont testnet (#2543)

## Issue Addressed

N/A

## Proposed Changes

Removes all configurations and hard-coded rules related to the deprecated Pyrmont testnet.

## Additional Info

Pyrmont is deprecated/will be shut down after being used for scenario testing, this PR removes configurations related to it.

Co-authored-by: Zachinquarantine <zachinquarantine@yahoo.com>

book/src/testnet-validator.md

@@ -1,7 +1,6 @@
 # Become a Testnet Validator
 
 [mainnet-validator]: ./mainnet-validator.md
-[pyrmont-launchpad]: https://pyrmont.launchpad.ethereum.org/
 [prater-launchpad]: https://prater.launchpad.ethereum.org/
 
 Joining an Eth2 testnet is a great way to get familiar with staking in Phase 0.  All users should
@@ -11,15 +10,12 @@ To join a testnet, you can follow the [Become an Eth2 Mainnet Validator][mainnet
 instructions but with a few differences:
 
 1. Use the appropriate Eth2 launchpad website:
-    - [Pyrmont][pyrmont-launchpad]
     - [Prater][prater-launchpad]
 1. Instead of `--network mainnet`, use the appropriate network flag:
-   - `--network pyrmont`: Pyrmont.
    - `--network prater`: Prater.
 1. Use a Goerli Eth1 node instead of a mainnet one:
    - For Geth, this means using `geth --goerli --http`.
 1. Notice that Lighthouse will store its files in a different directory by default:
-   - `~/.lighthouse/pyrmont`: Pyrmont.
    - `~/.lighthouse/prater`: Prater.
 
 >