Remove xcodeproj & xcshareddata ignores

[capnslipp]

The .gitignore file currently is ignoring xcodepoj-related files, which currently does nothing for the Then project because of the decision to go full SPM and remove all xcodeproj stuff (#55).

However, ignoring these files causes issues when Then's source is included in outer Git repo (via git-subtree or direct copying of project) such as when using Carthage or manual library management in conjunction with a generated xcodeproj (swift package generate-xcodeproj). Because Then's .gitignore is picked up by the enclosing repo, that prevents the generated xcodeproj or shared schemes from being able to be checked-in— which should arguably be under the discretion of the enclosing repo's build setup.  Removed the ignores for xcodeproj and xcshareddata to fix this, and removed the xcuserdata line too for good measure (again, if there's no xcodeproj in Then, there's no need to ignore it).

[capnslipp]

BTW, If a dev is working on the Then project itself and needs to generate an xcodeproj (rather than using Xcode 11's Swift package-project mode) but is afraid of accidentally committing it (or just hates non-clean working directories like I do), these ignores can be easily added to one's only local-repo-only in the .git/info/exclude file (same format a .gitignore; create the file if it doesn't exist; doesn't ever get sent to remote).