Remove SPM Manifest

[liamrosenfeld]

Summary

Apple is building SPM into Xcode so the manifest is no longer needed to future proof SPM compatibility

Review Checklist

• [x] I have responded to reviews and made changes where appropriate.
• [x] I have tested the code
• [x] I have updated comments / documentation related to the changes I made.
• [x] I have rebased my PR branch onto xi-mac/master.

[liamrosenfeld]

thank you!