[Explicit Module Builds] Remove dead/incorrect code for handling header inputs of binary module dependencies

As of swiftlang/swift#72067, we instead serialize `.h` inputs directly into binary `.swiftmodule` files, because their clients may not be able to use the dependnecies' corresponding `.pch` files due to a compilation context mismatch. The clients of such binary modules then consume the serialized `.h` files directly, and compile them, implicitly, using explicit module dependencies collected during scan of such header files.

Resolves rdar://131261765

Author: artemcm

Sources/SwiftDriver/ExplicitModuleBuilds/ExplicitDependencyBuildPlanner.swift

@@ -269,12 +269,6 @@ public typealias ExternalTargetModuleDetailsMap = [ModuleDependencyId: ExternalT
       if cas != nil && !prebuiltHeaderDependencyPaths.isEmpty {
         throw DependencyScanningError.unsupportedConfigurationForCaching("module \(dependencyModule.moduleName) has bridging header dependency")
       }
-
-      for headerDep in prebuiltHeaderDependencyPaths {
-        commandLine.appendFlags(["-Xcc", "-include-pch", "-Xcc"])
-        commandLine.appendPath(VirtualPath.lookup(headerDep.path))
-        inputs.append(TypedVirtualPath(file: headerDep.path, type: .pch))
-      }
     }
     for moduleArtifactInfo in clangDependencyArtifacts {
       let clangModulePath =