Truly decouple wallet from chainparams for -fallbackfee

Before it was 0 by default for main and 20000 for test and regtest.
Now it is 0 by default for all chains, thus there's no need to call Params().

Also now the default for main is properly documented

test/functional/test_framework/util.py

@@ -299,6 +299,7 @@ def initialize_datadir(dirname, n, chain):
         f.write("[{}]\n".format(chain_name_conf_section))
         f.write("port=" + str(p2p_port(n)) + "\n")
         f.write("rpcport=" + str(rpc_port(n)) + "\n")
+        f.write("fallbackfee=0.0002\n")
         f.write("server=1\n")
         f.write("keypool=1\n")
         f.write("discover=0\n")