docs: Update docs/elements.conf [skip ci]

Most of the config options here are activated by using chain=liquidtestnet.

Documentation for the config options can be found using the built-in elementsd help:

Example:

$ elementsd -help | grep -A1 con_max_block_sig_size

  -con_max_block_sig_size
	Max allowed witness data for the signed block header.

docs/elements.conf

@@ -9,24 +9,10 @@ txindex=1
 validatepegin=0
 anyonecanspendaremine=0
 initialfreecoins=2100000000000000
-con_dyna_deploy_start=0
-con_max_block_sig_size=150
-connect=liquid-testnet.blockstream.com:18892
 fallbackfee=0.00000100
 daemon=1
-con_has_parent_chain=0
-parentgenesisblockhash=NULL
-pubkeyprefix=36
-scriptprefix=19
-blindedprefix=23
-bech32_hrp=tex
-blech32_hrp=tlq
-pchmessagestart=410edd62
-dynamic_epoch_length=1000
-signblockscript=51210217e403ddb181872c32a0cd468c710040b2f53d8cac69f18dad07985ee37e9a7151ae
 rpcport=18884
 rpcuser=admin1
 rpcpassword=123
 rpcallowip=0.0.0.0/0
 rpcbind=0.0.0.0
-addnode=49.12.106.176:18891