Remove unused and deprecated import.

Author: whitequark

tests/test_serial.py

@@ -5,7 +5,6 @@
 from amaranth import *
 from amaranth.lib.data import StructLayout
 from amaranth.lib.fifo import SyncFIFO
-from amaranth.lib.io import pin_layout
 from amaranth.lib.wiring import *
 from amaranth.sim import *