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/wallet_encryption.py

@@ -134,13 +134,13 @@ class WalletEncryptionTest(BitcoinTestFramework):
             do_wallet_tool("-wallet=noprivs", f"-dumpfile={dumpfile_path}", "dump")
 
             # Modify the dump
-            with open(dumpfile_path, "r", encoding="utf-8") as f:
+            with open(dumpfile_path, "r") as f:
                 dump_content = f.readlines()
             # Drop the checksum line
             dump_content = dump_content[:-1]
             # Insert a valid mkey line. This corresponds to a passphrase of "pass".
             dump_content.append("046d6b657901000000,300dc926f3b3887aad3d5d5f5a0fc1b1a4a1722f9284bd5c6ff93b64a83902765953939c58fe144013c8b819f42cf698b208e9911e5f0c544fa300000000cc52050000\n")
-            with open(dumpfile_path, "w", encoding="utf-8") as f:
+            with open(dumpfile_path, "w") as f:
                 contents = "".join(dump_content)
                 f.write(contents)
                 checksum = hash256(contents.encode())
@@ -158,7 +158,7 @@ class WalletEncryptionTest(BitcoinTestFramework):
             # Make a new dump and check that there are no mkeys
             dumpfile_path = self.nodes[0].datadir_path / "noprivs_enc.dump"
             do_wallet_tool("-wallet=noprivs_enc", f"-dumpfile={dumpfile_path}", "dump")
-            with open(dumpfile_path, "r", encoding="utf-8") as f:
+            with open(dumpfile_path, "r") as f:
                 # Check there's nothing with an 'mkey' prefix
                 assert_equal(all([not line.startswith("046d6b6579") for line in f]), True)