[utils] Remove unused method.

The method was not used anywhere, and Workspace provides a better alternative which gives the full path instead of just the directory name.

Author: drodriguez

utils/swift_build_support/swift_build_support/products/product.py

@@ -30,11 +30,6 @@ def product_source_name(cls):
         """
         return cls.product_name()
 
-    @classmethod
-    def get_build_directory_name(cls, host_target):
-        return "{}-{}".format(cls.product_name(),
-                              host_target.name)
-
     @classmethod
     def is_build_script_impl_product(cls):
         """is_build_script_impl_product -> bool