[6.0] Remove Sendable conformance on Bundle from SwiftPM (#7653)

Cherry-pick of #7652.

**Explanation**: We shouldn't have two `Sendable` conformances on `Bundle` at the same time, and Foundation added this on their side in `main` (swiftlang/swift-corelibs-foundation#4963)
and `release/6.0` (swiftlang/swift-corelibs-foundation#4978).
**Scope**: isolated to bundle resources.
**Risk**: low, the corresponding change on `main` branch of
`swift-corelibs-foundation` was there for 3 weeks.
**Testing**: covered by existing tests and the compat suite.
**Issue**: rdar://129599679
**Reviewer**: @parkera, @bnbarham

Author: MaxDesiatov

Sources/Build/BuildDescription/SwiftTargetBuildDescription.swift

@@ -380,11 +380,6 @@ public final class SwiftTargetBuildDescription {
             """
             import Foundation
 
-            #if compiler(>=6.0)
-            extension Foundation.Bundle: @unchecked @retroactive Sendable {}
-            #else
-            extension Foundation.Bundle: @unchecked Sendable {}
-            #endif
             extension Foundation.Bundle {
                 static let module: Bundle = {
                     let mainPath = \(mainPathSubstitution)