No need to delete it.
From your new local repo create directory or remote simple the .git, if you want upload (without history) an existing repo:
rm -rf .git
Recreate the repos from the current content only
git init
git add .
git commit -m "Initial commit"
Then simply push to the github remote repos ensuring you overwrite history:
git push --force -u origin master
That will replace the history of your BitBucket master branch by the new one you have done locally.
Now, if you had pushed other branches before, you might want to delete them.
git push origin :oldBranch