gcode: Remove support for "G4 S1" style commands

The S parameter is not a standard and was not commonly used.  The P
parameter (delay specified in milliseconds) is the commonly accepted
g-code mechanism for dwelling.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

docs/Config_Changes.md

@@ -6,6 +6,10 @@ All dates in this document are approximate.
 
 # Changes
 
+20191202: Support for the undocumented "S" parameter of the "G4"
+command has been removed.  Replace any occurrences of S with the
+standard "P" parameter (the delay specified in milliseconds).
+
 20191126: The USB names have changed on micro-controllers with native
 USB support.  They now use a unique chip id by default (where
 available).  If an "mcu" config section uses a "serial" setting that

klippy/gcode.py

@@ -515,10 +515,7 @@ class GCodeParser:
         self.move_with_transform(self.last_position, self.speed)
     def cmd_G4(self, params):
         # Dwell
-        if 'S' in params:
-            delay = self.get_float('S', params, minval=0.)
-        else:
-            delay = self.get_float('P', params, 0., minval=0.) / 1000.
+        delay = self.get_float('P', params, 0., minval=0.) / 1000.
         self.toolhead.dwell(delay)
     def cmd_G28(self, params):
         # Move to origin