Revert "Sema: Diagnose @backDeployed functions with missing bodies in swiftinterfaces"

[DougGregor]

Reverts #78327. It's breaking the macOS toolchain build with

/Users/ec2-user/jenkins/workspace/swift-PR-toolchain-macos/branch-main/buildbot_osx/swift-macosx-x86_64/lib/swift/xros/Swift.swiftmodule/arm64-apple-xros.private.swiftinterface:2371:2: error: '@backDeployed' requires that global function '_diagnoseUnavailableCodeReached()' have a body
 2369 | @usableFromInline
 2370 | @inline(never) internal func _diagnoseUnexpectedEnumCase<SwitchedValue>(type: SwitchedValue.Type) -> Swift.Never
 2371 | @backDeployed(before: macOS 14.0, iOS 17.0, watchOS 10.0, tvOS 17.0)
      |  `- error: '@backDeployed' requires that global function '_diagnoseUnavailableCodeReached()' have a body
 2372 | @usableFromInline
 2373 | @inline(never) @_semantics("unavailable_code_reached") internal func _diagnoseUnavailableCodeReached() -> Swift.Never

It's not that this PR is wrong, it's that it uncovered an existing problem.

[DougGregor]

@swift-ci please test

[DougGregor]

@swift-ci please build toolchain

[shahmishal]

Merged to unblock nightly toolchain.