Remove Git repository, but keep all files

Just remove (or rename) the .git subdirectory. That subdirectory is the whole of the repository. This will not delete your files, but it will delete their history.

Additionally, there might be files like .gitignore and .gitattributes that are not necessary anymore.

Make sure that all your files are present in the working tree before removing/renaming .git.

Your repository resides in the .git directory, you can delete it easily. But if only the prompt disturbs you, it is enough to search where PROMPT gets new value in your environment and modify it to something else.