Remove some unused exceptions

src/textual/color.py

@@ -528,7 +528,6 @@ class Color(NamedTuple):
 # Color constants
 WHITE = Color(255, 255, 255)
 BLACK = Color(0, 0, 0)
-TRANSPARENT = Color(0, 0, 0, 0)
 
 
 def rgb_to_lab(rgb: Color) -> Lab:

src/textual/dom.py

@@ -75,10 +75,6 @@ class NoScreen(DOMError):
     pass
 
 
-class NoParent(Exception):
-    pass
-
-
 @rich.repr.auto
 class DOMNode(MessagePump):
     """The base class for object that can be in the Textual DOM (App and Widget)"""

src/textual/message_pump.py

@@ -29,10 +29,6 @@ if TYPE_CHECKING:
     from .app import App
 
 
-class NoParent(Exception):
-    pass
-
-
 class CallbackError(Exception):
     pass