[tests] remove bn2bin()

It's only called in one place.

test/functional/test_framework/bignum.py

@@ -9,29 +9,29 @@ endian format.
 
 This file is copied from python-bitcoinlib.
 """
-def bn2bin(v):
-    """Convert a number to a byte array."""
-    s = bytearray()
-    bytes_len = (v.bit_length() + 7) // 8
-    for i in range(bytes_len, 0, -1):
-        s.append((v >> ((i - 1) * 8)) & 0xff)
-    return s
-
 def bn2mpi(v):
     """Convert number to MPI format, without the sign byte."""
+    # The top bit is used to indicate the sign of the number. If there
+    # isn't a spare bit in the bit length, add an extension byte.
     have_ext = False
+    ext = bytearray()
     if v.bit_length() > 0:
         have_ext = (v.bit_length() & 0x07) == 0
+        ext.append(0)
 
+    # Is the number negative?
     neg = False
     if v < 0:
         neg = True
         v = -v
 
-    ext = bytearray()
-    if have_ext:
-        ext.append(0)
-    v_bin = bn2bin(v)
+    # Convert the int to bytes
+    v_bin = bytearray()
+    bytes_len = (v.bit_length() + 7) // 8
+    for i in range(bytes_len, 0, -1):
+        v_bin.append((v >> ((i - 1) * 8)) & 0xff)
+
+    # Add the sign bit if necessary
     if neg:
         if have_ext:
             ext[0] |= 0x80