Move directivesRemovedFromContent to BlockDirective

While looking at swiftlang#982, I found that there's a better place to keep the list of directive names which we consider removed from content -- we already had a file with lists of directive names so it'll be clearer to keep all of them in the same file.

Sources/SwiftDocC/Model/DocumentationMarkup.swift

@@ -154,7 +154,7 @@ struct DocumentationMarkup {
                         // Found deprecation notice in the abstract.
                         deprecation = MarkupContainer(directive.children)
                         return
-                    } else if Self.directivesRemovedFromContent.contains(directive.name) {
+                    } else if BlockDirective.directivesRemovedFromContent.contains(directive.name) {
                         // These directives don't affect content so they shouldn't break us out of
                         // the automatic abstract section.
                         return

Sources/SwiftDocC/Utility/MarkupExtensions/BlockDirectiveExtensions.swift

@@ -54,4 +54,11 @@ extension BlockDirective {
         Volume.directiveName,
         XcodeRequirement.directiveName
     ]
+
+    static let directivesRemovedFromContent: [String] = [
+        Comment.directiveName,
+        Metadata.directiveName,
+        Options.directiveName,
+        Redirect.directiveName,
+    ]
 }