Remove ill-defined, broken, and unused ptr_name function

In case this is needed, the commit can be reverted.

See discussion in #150.

pycroft/lib/net.py

@@ -6,7 +6,6 @@
 ~~~~~~~~~~~~~~~
 """
 import typing as t
-from itertools import islice
 
 import netaddr
 from sqlalchemy import func, and_, cast
@@ -100,20 +99,6 @@ def get_subnets_with_usage() -> list[tuple[Subnet, SubnetUsage]]:
     ]
 
 
-def ptr_name(network: netaddr.IPNetwork, ip_address: netaddr.IPAddress) -> str:
-    hostbits = network.max_prefixlen - network.prefixlen
-    if ip_address.version == 4:
-        num_octets = min((hostbits + 7 // 8), 1)
-        reversed_octets = reversed(ip_address.exploded.split('.'))
-        return '.'.join(islice(reversed_octets, num_octets))
-
-    if ip_address.version == 6:
-        num_chars = min((hostbits + 3 // 4), 1)
-        reversed_chars = reversed(ip_address.exploded.replace(':', ''))
-        return '.'.join(islice(reversed_chars, num_chars))
-    raise TypeError()
-
-
 def delete_ip(session: Session, ip: netaddr.IPAddress) -> None:
     # TODO use proper `delete` statement
     session.delete(IP.q.filter_by(address=ip).first())