[ZEPPELIN-5529] Removal of the pegdown parser because it's EOL

### What is this PR for?
This PR removes pegdown, because it's EOL

### What type of PR is it?
 - Improvement

### What is the Jira issue?
* https://issues.apache.org/jira/browse/ZEPPELIN-5529

### How should this be tested?
* CI

### Questions:
* Does the licenses files need update? No
* Is there breaking changes for older versions? No
* Does this needs documentation? No

Author: Philipp Dallig <philipp.dallig@gmail.com>

Closes apache#4229 from Reamer/remove_pegdown and squashes the following commits:

df7a693 [Philipp Dallig] Removal of the pegdown parser because it reached the end of its life a long time ago.

markdown/pom.xml

@@ -46,12 +46,6 @@
       <version>${markdown4j.version}</version>
     </dependency>
 
-    <dependency>
-      <groupId>org.pegdown</groupId>
-      <artifactId>pegdown</artifactId>
-      <version>${pegdown.version}</version>
-    </dependency>
-
     <dependency>
       <groupId>org.apache.httpcomponents</groupId>
       <artifactId>httpclient</artifactId>

markdown/src/main/java/org/apache/zeppelin/markdown/Markdown.java

@@ -46,13 +46,6 @@ public class Markdown extends Interpreter {
    * Markdown Parser Type.
    */
   public enum MarkdownParserType {
-    PEGDOWN {
-      @Override
-      public String toString() {
-        return PARSER_TYPE_PEGDOWN;
-      }
-    },
-
     MARKDOWN4j {
       @Override
       public String toString() {
@@ -70,7 +63,6 @@ public String toString() {
   }
 
   public static final String MARKDOWN_PARSER_TYPE = "markdown.parser.type";
-  public static final String PARSER_TYPE_PEGDOWN = "pegdown";
   public static final String PARSER_TYPE_MARKDOWN4J = "markdown4j";
   public static final String PARSER_TYPE_FLEXMARK = "flexmark";
 
@@ -81,9 +73,7 @@ public Markdown(Properties property) {
   public static MarkdownParser createMarkdownParser(String parserType) {
     LOGGER.debug("Creating {} markdown interpreter", parserType);
 
-    if (MarkdownParserType.PEGDOWN.toString().equals(parserType)) {
-      return new PegdownParser();
-    } else if (MarkdownParserType.FLEXMARK.toString().equals(parserType)) {
+    if (MarkdownParserType.FLEXMARK.toString().equals(parserType)) {
       return new FlexmarkParser();
     } else {
       // default parser

markdown/src/main/resources/interpreter-setting.json

@@ -8,7 +8,7 @@
         "envName": "MARKDOWN_PARSER_TYPE",
         "propertyName": "markdown.parser.type",
         "defaultValue": "flexmark",
-        "description": "Markdown Parser Type. Available values: pegdown, markdown4j, flexmark. Default = flexmark",
+        "description": "Markdown Parser Type. Available values: markdown4j, flexmark. Default = flexmark",
         "type": "string"
       }
     },