[tests] remove bn_bytes() function

It is one line and is called in one place.

test/functional/test_framework/bignum.py

@@ -9,17 +9,12 @@ endian format.
 
 This file is copied from python-bitcoinlib.
 """
-def bn_bytes(v):
-    """Return number of bytes in integer representation of v."""
-    return (v.bit_length() + 7) // 8
-
 def bn2bin(v):
     """Convert a number to a byte array."""
     s = bytearray()
-    i = bn_bytes(v)
-    while i > 0:
+    bytes_len = (v.bit_length() + 7) // 8
+    for i in range(bytes_len, 0, -1):
         s.append((v >> ((i - 1) * 8)) & 0xff)
-        i -= 1
     return s
 
 def bn2mpi(v):