drop support for full @user@instance handles

This is unnecessary because it only supports local users anyway
plus no finger client will send the hostname

fingerd.py

@@ -17,23 +17,14 @@ with open('fingerd.toml') as f:
 
 async def handle_finger(reader, writer):
 	try:
-		data = await reader.read(
-			len('@') * 2
-			+ MASTODON_USERNAME_MAX_LENGTH
-			+ len(config['instance_name'])
-		)
-		handle = data.decode().rstrip()
+		data = await reader.read(MASTODON_USERNAME_MAX_LENGTH)
+		username = data.decode().rstrip()
 
-		if not handle:
+		if not username:
 			# `finger @instance.example`
 			await handle_finger_instance(reader, writer)
 			return
 
-		username = handle.removeprefix('@').removesuffix('@' + config['instance_name'])
-		if '@' in username:
-			writer.write(b'Error: user must be local to this instance\n')
-			return
-
 		resp = await db.get().fetchval(
 			'SELECT note FROM accounts WHERE username = $1 AND domain IS NULL',
 			username,