These solutions are way too difficult. The problem is that you have removed the project from filesystem but SVN still thinks they are there.
Detaching project from SVN will work, the same for removing .svn folders BUT that is going to destroy your repository!
It is much easier to open console, browse to your folder ("cd /pathToYourFolder") and type the following command:
svn delete nameOfMissingFile
If the name of your missing file includes the @ character (eg: retina-specific artwork), ensure the file name terminates with the @ symbol:
svn delete nameOfMissingFile@2x.png@
For GIT repositories:
git rm nameOfMissingFile
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