hdl.ir: Elaboratable does not need ABCMeta as its metaclass.

This was introduced in commit 44711b7, and was never used within
Amaranth itself. While technically a breaking change I think this
will not cause enough breakage to warrant a deprecation cycle
(nor can we make this a deprecation this without a lot of work).

Author: whitequark

amaranth/hdl/ir.py

@@ -20,7 +20,7 @@ class UnusedElaboratable(UnusedMustUse):
     _MustUse__silence = True
 
 
-class Elaboratable(MustUse, metaclass=ABCMeta):
+class Elaboratable(MustUse):
     _MustUse__warning = UnusedElaboratable