Undo 5.7 package files, not needed yet (#232)

* Undo 5.7 package files, not needed yet

* no need for xcrun in docs preview script

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.7
+// swift-tools-version:5.6
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the Swift Logging API open source project

Package@swift-5.6.swift

@@ -1,36 +0,0 @@
-// swift-tools-version:5.6
-//===----------------------------------------------------------------------===//
-//
-// This source file is part of the Swift Logging API open source project
-//
-// Copyright (c) 2018-2019 Apple Inc. and the Swift Logging API project authors
-// Licensed under Apache License v2.0
-//
-// See LICENSE.txt for license information
-// See CONTRIBUTORS.txt for the list of Swift Logging API project authors
-//
-// SPDX-License-Identifier: Apache-2.0
-//
-//===----------------------------------------------------------------------===//
-
-import PackageDescription
-
-let package = Package(
-    name: "swift-log",
-    products: [
-        .library(name: "Logging", targets: ["Logging"]),
-    ],
-    dependencies: [
-        .package(url: "https://github.com/apple/swift-docc-plugin", from: "1.0.0"),
-    ],
-    targets: [
-        .target(
-            name: "Logging",
-            dependencies: []
-        ),
-        .testTarget(
-            name: "LoggingTests",
-            dependencies: ["Logging"]
-        ),
-    ]
-)

scripts/preview_docc.sh

@@ -13,4 +13,4 @@
 ##
 ##===----------------------------------------------------------------------===##
 
-xcrun swift package --disable-sandbox preview-documentation --target Logging
+swift package --disable-sandbox preview-documentation --target Logging