T7576: Remove unnecessary code for checking dirty build status

```
vyos@r15:~$ show version | match dir
Build commit ID:  d8038de-dirty
```
Having uncommited files in vyos-build is completely normal:
custom configs, EULA files in official release builds,
anything else that flavor files may include.

However, many people are not familiar with the specific
meaning of "dirty" in git or with the build process and assume
that "dirty" means something went wrong with the image build.

Author: alexandr-san4ez

scripts/image-build/build-vyos-image

@@ -409,9 +409,6 @@ if __name__ == "__main__":
             repo = git.Repo('.', search_parent_directories=True)
             # Retrieve the Git commit ID of the repository, 14 charaters will be sufficient
             build_git = repo.head.object.hexsha[:14]
-            # If somone played around with the source tree and the build is "dirty", mark it
-            if repo.is_dirty():
-                build_git += "-dirty"
 
             # Retrieve git branch name or current tag
             # Building a tagged release might leave us checking out a git tag that is not the tip of a named branch (detached HEAD)