Remove question about version validation place

In issue theupdateframework#1418 in this comment:
theupdateframework#1418 (comment)
I summarized the discussion we had with the participants in this issue.
In summary: no additional changes are needed for "version" validation
considering there is "bump_version()" function for that.

If we won't be adding "version" validation elsewhere we can keep it
the way it is.

Signed-off-by: Martin Vrachev <[email protected]>

tuf/api/metadata.py

@@ -293,7 +293,6 @@ def __init__(
         self.spec_version = spec_version
         self.expires = expires
 
-        # TODO: Should we separate data validation from constructor?
         if version <= 0:
             raise ValueError(f"version must be > 0, got {version}")
         self.version = version