contrib: Remove confusing and redundant encoding from IO

The encoding arg is confusing, because it is not applied consistently
for all IO.

Also, it is useless, as the majority of files are ASCII encoded, which
are fine to encode and decode with any mode.

Moreover, UTF-8 is already required for most scripts to work properly,
so setting the encoding twice is redundant.

So remove the encoding from most IO. It would be fine to remove from all
IO, however I kept it for two files:

* contrib/asmap/asmap-tool.py: This specifically looks for utf-8
  encoding errors, so it makes sense to sepecify the utf-8 encoding
  explicitly.
* test/functional/test_framework/test_node.py: Reading the debug log in
  text mode specifically counts the utf-8 characters (not bytes), so it
  makes sense to specify the utf-8 encoding explicitly.

test/functional/test_framework/util.py

@@ -528,7 +528,7 @@ def write_config(config_path, *, n, chain, extra_config="", disable_autoconnect=
     else:
         chain_name_conf_arg = chain
         chain_name_conf_section = chain
-    with open(config_path, 'w', encoding='utf8') as f:
+    with open(config_path, 'w') as f:
         if chain_name_conf_arg:
             f.write("{}=1\n".format(chain_name_conf_arg))
         if chain_name_conf_section:
@@ -594,7 +594,7 @@ def get_temp_default_datadir(temp_dir: pathlib.Path) -> tuple[dict, pathlib.Path
 
 
 def append_config(datadir, options):
-    with open(os.path.join(datadir, "bitcoin.conf"), 'a', encoding='utf8') as f:
+    with open(os.path.join(datadir, "bitcoin.conf"), 'a') as f:
         for option in options:
             f.write(option + "\n")
 
@@ -603,7 +603,7 @@ def get_auth_cookie(datadir, chain):
     user = None
     password = None
     if os.path.isfile(os.path.join(datadir, "bitcoin.conf")):
-        with open(os.path.join(datadir, "bitcoin.conf"), 'r', encoding='utf8') as f:
+        with open(os.path.join(datadir, "bitcoin.conf"), 'r') as f:
             for line in f:
                 if line.startswith("rpcuser="):
                     assert user is None  # Ensure that there is only one rpcuser line
@@ -612,7 +612,7 @@ def get_auth_cookie(datadir, chain):
                     assert password is None  # Ensure that there is only one rpcpassword line
                     password = line.split("=")[1].strip("\n")
     try:
-        with open(os.path.join(datadir, chain, ".cookie"), 'r', encoding="ascii") as f:
+        with open(os.path.join(datadir, chain, ".cookie"), 'r') as f:
             userpass = f.read()
             split_userpass = userpass.split(':')
             user = split_userpass[0]