Its better to perform this over Master branch
Edit .gitignore file. Add the below line in it.
.idea
Remove .idea folder from remote repo. using below command.
git rm -r --cached .idea
For more info. reference: Removing Files from a Git Repository Without Actually Deleting Them
Stage .gitignore file. Using below command
git add .gitignore
Commit
git commit -m 'Removed .idea folder'
Push to remote
git push origin master