remove unused config CURRENT_VERSION (#21832)

as API module's version code refactored into pyproject.toml file in refactor: define the Dify project version in pyproject.toml #20910, the deprecated CURRENT_VERSION is no longger used and should be removed.

api/configs/packaging/__init__.py

     Packaging build information
     """
 
-    CURRENT_VERSION: str = Field(
-        description="Dify version",
-        default="1.5.1",
-    )
-
     COMMIT_SHA: str = Field(
         description="SHA-1 checksum of the git commit used to build the app",
         default="",