drop class method identifier from docs

For example, this changes

classmethod sound.beeps(number)
   Description

into

sound.beeps(number)
   Description

pybricks/_common.py

@@ -211,10 +211,12 @@ class Motor():
 class Display():
     """Show images or text on a display."""
 
+    @staticmethod
     def clear(self):
         """Clear everything on the display."""
         pass
 
+    @staticmethod
     def text(self, text, coordinate=None):
         """Display text.
 
@@ -225,6 +227,7 @@ class Display():
         """
         pass
 
+    @staticmethod
     def image(self, file_name, alignment=Align.CENTER, coordinate=None, clear=True):
         """image(file_name, alignment=Align.CENTER, coordinate=None, clear=True)
 
@@ -245,6 +248,7 @@ class Display():
 class Speaker():
     """Play beeps and sound files using a speaker."""
 
+    @staticmethod
     def beep(self, frequency=500, duration=100, volume=30):
         """Play a beep/tone.
 
@@ -255,6 +259,7 @@ class Speaker():
         """
         pass
 
+    @staticmethod
     def beeps(self, number):
         """Play a number of default beeps with a brief pause in between.
 
@@ -264,6 +269,7 @@ class Speaker():
         """
         pass
 
+    @staticmethod
     def file(self, file_name, volume=100):
         """Play a sound file.
 
@@ -279,14 +285,17 @@ class Speaker():
 class Light():
     """Control an LED."""
 
+    @staticmethod
     def on(self):
         """Turn on the light at maximum brightness."""
         pass
 
+    @staticmethod
     def off(self):
         """Turn off the light."""
         pass
 
+    @staticmethod
     def brightness(self, brightness):
         """Set the perceived brightness of the light.
 
@@ -299,6 +308,7 @@ class Light():
 class ColorLight():
     """Control an RGB LED."""
 
+    @staticmethod
     def color(color):
         """Turn on the light at the specified color.
 
@@ -307,10 +317,12 @@ class ColorLight():
         """
         pass
 
+    @staticmethod
     def off(self):
         """Turn off the light."""
         pass
 
+    @staticmethod
     def brightness(self, red, green, blue):
         """Set the perceived brightness of the red, green, and blue light.
 
@@ -324,6 +336,7 @@ class ColorLight():
 class Battery():
     """Get the status of a battery."""
 
+    @staticmethod
     def voltage(self):
         """Get the voltage of the battery.
 
@@ -332,6 +345,7 @@ class Battery():
         """
         pass
 
+    @staticmethod
     def current(self):
         """Get the current supplied by the battery.