Plugins: Remove leniency for missing plugins dir (#24173)

This leniency was left in after plugin installer refactoring for 2.0
because some tests still relied on it. However, the need for this
leniency no longer exists.

Author: rjernst

distribution/tools/plugin-cli/src/main/java/org/elasticsearch/plugins/InstallPluginCommand.java

@@ -206,11 +206,6 @@ void execute(Terminal terminal, String pluginId, boolean isBatch, Environment en
         if (pluginId == null) {
             throw new UserException(ExitCodes.USAGE, "plugin id is required");
         }
-        // TODO: remove this leniency!! is it needed anymore?
-        if (Files.exists(env.pluginsFile()) == false) {
-            terminal.println("Plugins directory [" + env.pluginsFile() + "] does not exist. Creating...");
-            Files.createDirectory(env.pluginsFile());
-        }
 
         Path pluginZip = download(terminal, pluginId, env.tmpFile());
         Path extractedZip = unzip(pluginZip, env.pluginsFile());