removes dead links from intro-to-agile

agile-development/intro-to-agile.md

@@ -60,7 +60,6 @@ The benefits of Agile software development includes:
 - [Video: Explaining Scrum in less than 120 seconds](https://www.youtube.com/watch?v=WxiuE-1ujCM)
 - [Blog Post: What is a user story?(4 min read)](https://www.mountaingoatsoftware.com/agile/user-stories)
 - [Blog Post: What Is a Kanban Board?(5 min read)](https://leankit.com/learn/kanban/kanban-board/)
-- [Blog Post: What is a retrospective?(2 min read)](http://finding-marbles.com/retr-o-mat/what-is-a-retrospective/)
 
 ### Definition of Agile
 
@@ -191,7 +190,6 @@ Process:
 - [Blog Post: What is a user story? (4 min read)](https://www.mountaingoatsoftware.com/agile/user-stories)
 - [Time-boxing](https://www.agilealliance.org/glossary/iteration)
 - [Blog Post: What Is a Kanban Board? (5 min read)](https://leankit.com/learn/kanban/kanban-board/)
-- [Blog Post: What is a retrospective? (2 min read)](http://finding-marbles.com/retr-o-mat/what-is-a-retrospective/)
 
 Technical:
 
@@ -251,4 +249,3 @@ Incorporate the following in personal study:
 - [Wiki Article: eXtreme Programming (XP)(15 min read)](https://en.wikipedia.org/wiki/Extreme_programming)
 - [Wiki Article: Feature Driven Delopment (FDD)(8 min read)](https://en.wikipedia.org/wiki/Feature-driven_development)
 - [Martin Fowler's blog: A website on building software effectively(3 min read) ](https://martinfowler.com/)
-- [Finding Marbles blog: How work can be – Stories of agile & lean - Corinna Baldauf(10 min read)](http://finding-marbles.com/)