chore: remove xud.conf

[reliveyy]

Partially resolving #405

This PR replaces xud.conf file with a bunch of command-line options in entrypoint.sh. It also removes xud script option --xud.preserve-config. In a legacy environment, the old xud.conf will be renamed to previous-xud.{timestamp}.conf.

How to test?

The expected behaviour is not breaking anything about the xud service.

[raladev]

• simnet-lndltc throws connection error during initialization

Syncing light clients:
┌─────────┬─────────────────────────────────────────────────────┐
│ SERVICE │ STATUS                                              │
├─────────┼─────────────────────────────────────────────────────┤
│ lndbtc  │ Syncing 99.91% (116000/116094)                      │
├─────────┼─────────────────────────────────────────────────────┤
│ lndltc  │ Syncing 99.83% (116000/116193)                      │
└─────────┴─────────────────────────────────────────────────────┘

Do you want to create a new xud environment or restore an existing one?
1) Create New
2) Restore Existing
Please choose: 1
You are creating an xud node key and underlying wallets. All will be secured by a single password provided below.

Enter a password: 
Re-enter password: 

Error: 13 INTERNAL: could not initialize lnd-LTC: 14 UNAVAILABLE: failed to connect to all addresses
Do you want to create a new xud environment or restore an existing one?
1) Create New
2) Restore Existing
Please choose: 

[raladev]

• something wrong with backup path. lnd backup files are not created and xud log contains err:
  
  

[raladev]

• testnet setup does not work because of wrong port choosing by utils