Remove definition of Mergeable from dbt/contracts/util

Although we've removed the definition of `Mergeable` we've ensured the
import paths are still available. We do this because this is under
`contracts`, and the sudden disappearance from the import path might
cause issues for community members using dbt-core as a library.

Ideally we'd define a `Mergeable` class here that inherits the
`dbt-common` definition and raise a deprecation warning on instantiation.
However, we don't have an established strategy to do so.

core/dbt/contracts/util.py

@@ -1,8 +1,9 @@
-import dataclasses
 from typing import List, Any, Tuple
 
 from dbt_common.dataclass_schema import ValidatedStringMixin, ValidationError
-from dbt_common.contracts.util import Replaceable
+
+# Leave imports of `Mergeable` and `Replaceable` to preserve import paths
+from dbt_common.contracts.util import Mergeable, Replaceable  # noqa:F401
 
 
 SourceKey = Tuple[str, str]
@@ -24,22 +25,6 @@ class Foo:
     return []
 
 
-class Mergeable(Replaceable):
-    def merged(self, *args):
-        """Perform a shallow merge, where the last non-None write wins. This is
-        intended to merge dataclasses that are a collection of optional values.
-        """
-        replacements = {}
-        cls = type(self)
-        for arg in args:
-            for field in dataclasses.fields(cls):
-                value = getattr(arg, field.name)
-                if value is not None:
-                    replacements[field.name] = value
-
-        return self.replace(**replacements)
-
-
 class Identifier(ValidatedStringMixin):
     """Our definition of a valid Identifier is the same as what's valid for an unquoted database table name.