Delete unnecessary references of files in Xcode

[pixyzehn]

[nits]
I found unnecessary references of files in Xcode like this. I'm not sure these are needed or not. Please check my deletions.
I deleted 3 files references in this PR and already checked build and tests are passed.

[mono0926]

Thanks, those are ignored, but it seems that swift package generate-xcodeproj picked up them…
I'll be careful that.

[pixyzehn]

Thank you for checking!

It may be better to add *.xcodeproj in the .gitignore file like https://github.com/apple/swift-package-manager/blob/master/.gitignore#L10 because I have a lot of differences when executing swift package generate-xcodeproj.
For development, we can develop like this https://github.com/apple/swift-package-manager/blob/master/Documentation/Development.md#developing-using-xcode.

It's just my idea, please introduce them if you like👍

[pixyzehn]

#30
Thanks:)