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/crypto/ellswift.py

@@ -134,7 +134,7 @@ class TestFrameworkEllSwift(unittest.TestCase):
     def test_elligator_encode_testvectors(self):
         """Implement the BIP324 test vectors for ellswift encoding (read from xswiftec_inv_test_vectors.csv)."""
         vectors_file = os.path.join(os.path.dirname(os.path.realpath(__file__)), 'xswiftec_inv_test_vectors.csv')
-        with open(vectors_file, newline='', encoding='utf8') as csvfile:
+        with open(vectors_file, newline='') as csvfile:
             reader = csv.DictReader(csvfile)
             for row in reader:
                 u = FE.from_bytes(bytes.fromhex(row['u']))
@@ -150,7 +150,7 @@ class TestFrameworkEllSwift(unittest.TestCase):
     def test_elligator_decode_testvectors(self):
         """Implement the BIP324 test vectors for ellswift decoding (read from ellswift_decode_test_vectors.csv)."""
         vectors_file = os.path.join(os.path.dirname(os.path.realpath(__file__)), 'ellswift_decode_test_vectors.csv')
-        with open(vectors_file, newline='', encoding='utf8') as csvfile:
+        with open(vectors_file, newline='') as csvfile:
             reader = csv.DictReader(csvfile)
             for row in reader:
                 encoding = bytes.fromhex(row['ellswift'])