diskdef: Remove step= option

1. It's confusing when used with "gw convert"
2. It doesn't really have any place being used in a diskdef in
   the first place

src/greaseweazle/codec/codec.py

@@ -83,7 +83,6 @@ class DiskDef:
     def __init__(self) -> None:
         self.cyls: Optional[int] = None
         self.heads: Optional[int] = None
-        self.step = 1
         self.track_map: Dict[Tuple[int,int],TrackDef] = dict()
 
     def add_param(self, key: str, val: str) -> None:
@@ -95,10 +94,6 @@ def add_param(self, key: str, val: str) -> None:
             n = int(val)
             error.check(1 <= n <= 2, '%s out of range' % key)
             self.heads = n
-        elif key == 'step':
-            n = int(val)
-            error.check(1 <= n <= 4, '%s out of range' % key)
-            self.step = n
         else:
             raise error.Fatal('unrecognised disk option: %s' % key)
 
@@ -114,8 +109,6 @@ def trackset(self):
         s += ':h=0'
         if self.heads > 1:
             s += '-' + str(self.heads-1)
-        if self.step > 1:
-            s += ':step=' + str(self.step)
         return s
 
     def mk_track(self, cyl: int, head: int) -> Optional[codec.Codec]:

src/greaseweazle/data/diskdefs.cfg

@@ -6,8 +6,6 @@
 # the matching "end". Each disk definition must contain the following:
 #  cyls: Number of cylinders (1-255)
 #  heads: Number of heads/sides (1-2)
-# Also, optionally:
-#  step: Number of physical drive steps per image step (1-4)
 #
 # Each non-empty track in a disk requires a track definition:
 # "tracks <track-list> <format>"