Fix a few recently introduced code warnings (#1074)

* Fix warnings about `var` that's not mutated in test

* Fix warning about unused variable

Author: d-ronnqvist

Sources/SwiftDocC/Infrastructure/Symbol Graph/SymbolGraphLoader.swift

@@ -297,7 +297,6 @@ struct SymbolGraphLoader {
     /// If the bundle defines default availability for the symbols in the given symbol graph
     /// this method adds them to each of the symbols in the graph.
     private func addDefaultAvailability(to symbolGraph: inout SymbolGraph, moduleName: String) {
-        let selector = UnifiedSymbolGraph.Selector(forSymbolGraph: symbolGraph)
         // Check if there are defined default availabilities for the current module
         if let defaultAvailabilities = bundle.info.defaultAvailability?.modules[moduleName],
             let platformName = symbolGraph.module.platform.name.map(PlatformName.init) {

Tests/SwiftDocCTests/Infrastructure/SymbolGraph/SymbolGraphLoaderTests.swift

@@ -1738,7 +1738,7 @@ class SymbolGraphLoaderTests: XCTestCase {
              }
             """
         )
-        var infoPlist = """
+        let infoPlist = """
         <plist version="1.0">
         <dict>
             <key>CDAppleDefaultAvailability</key>
@@ -1769,7 +1769,7 @@ class SymbolGraphLoaderTests: XCTestCase {
         let infoPlistURL = targetURL.appendingPathComponent("Info.plist")
         try infoPlist.write(to: infoPlistURL, atomically: true, encoding: .utf8)
         // Load the bundle & reference resolve symbol graph docs
-        var (_, _, context) = try loadBundle(from: targetURL)
+        let (_, _, context) = try loadBundle(from: targetURL)
         guard let availability = (context.documentationCache["c:@F@Bar"]?.semantic as? Symbol)?.availability?.availability else {
             XCTFail("Did not find availability for symbol 'c:@F@Bar'")
             return