[Xcode 11.4] Remove -sdk_version from linker flags

Created by: tjwio

Background

It seems like Xcode 11.4 linker doesn't allow -sdk_version and -platform_version or -<platform>_version_min flags to be passed together or it throws this warning:

ld: warning: -sdk_version and -platform_version are not compatible, ignoring -sdk_version

Couple of things worth noting:

1. same flags passed in Xcode 11.3 and below doesn't throw this warning
2. normal xcodebuild command doesn't pass -sdk_version at all and seemingly only relies on -isysroot and -platform_version
3. I couldn't find any references on llvm's or apple's llvm-project related to -sdk_version, but did see this change for Xcode 11, passing the new platform_version by default so it might make sense that they're deprecating -sdk_version usage in 11.4 https://reviews.llvm.org/D71579

Changes

Remove -sdk_version from linker flags. It's seemingly not being used anymore and causing warnings with -platform_version.

Notes

Mostly opening this PR to start the discussion and get more info from people who are more knowledgable in this area. I couldn't find anything related to changes to using -sdk_version flag in llvm from 11.3 to 11.4, but it's throwing warnings when using it now in 11.4 so something must've changed. Also, not an expert on the usage of the flag but I do know it's seemingly not being used in vanilla xcodebuilds, so I wanted to get more info on that flag and why it's being passed in the first place.