ttl: remove timestamp function

The general idea is that functions that work with absolute timestamps exist only in machine units versions, to help prevent floating point losses of precision. Time differences should be computed in machine units and then converted, e.g. mu_to_seconds(t2-t1).

This function would have had problems after ~50 days of running the device.

artiq/coredevice/ttl.py

@@ -203,16 +203,6 @@ class TTLInOut:
         """
         return syscall("ttl_get", self.channel, self.i_previous_timestamp)
 
-    @kernel
-    def timestamp(self):
-        """Poll the RTIO input and returns an event timestamp (in seconds),
-        according to the gating.
-
-        If the gate is permanently closed, returns a negative value.
-        """
-        return mu_to_seconds(
-            syscall("ttl_get", self.channel, self.i_previous_timestamp))
-
 
 class TTLClockGen:
     """RTIO TTL clock generator driver.