lib.fifo: remove SyncFIFO.replace.

This obscure functionality was likely only ever used in old MiSoC
code, and doesn't justify the added complexity. It was also not
provided (and could not be reasonably provided) in SyncFIFOBuffered,
which made its utility extremely marginal.

nmigen/test/compat/test_fifo.py

@@ -36,21 +36,3 @@ class SyncFIFOCase(SimCase, unittest.TestCase):
                     self.assertEqual((yield self.tb.dut.dout[32:]), i*2)
                 yield
         self.run_with(gen())
-
-    def test_replace(self):
-        seq = [x for x in range(20) if x % 5]
-        def gen():
-            for cycle in count():
-                yield self.tb.dut.we.eq(cycle % 2 == 0)
-                yield self.tb.dut.re.eq(cycle % 7 == 0)
-                yield self.tb.dut.replace.eq(
-                    (yield self.tb.dut.din[:32]) % 5 == 1)
-                if (yield self.tb.dut.readable) and (yield self.tb.dut.re):
-                    try:
-                        i = seq.pop(0)
-                    except IndexError:
-                        break
-                    self.assertEqual((yield self.tb.dut.dout[:32]), i)
-                    self.assertEqual((yield self.tb.dut.dout[32:]), i*2)
-                yield
-        self.run_with(gen())