Remove broken Essential SQL link (#13060)

Author: CodeVishal-17

books/free-programming-books-langs.md

@@ -2528,7 +2528,6 @@ Books on general-purpose programming that don't focus on a specific language are
 ### SQL (implementation agnostic)
 
 * [Developing Time-Oriented Database Applications in SQL](https://www2.cs.arizona.edu/~rts/tdbbook.pdf) - Richard T. Snodgrass (PDF)
-* [Essential SQL](https://www.programming-books.io/essential/sql) - Krzysztof Kowalczyk and  Stack Overflow Documentation project (HTML)
 * [How to Design Structured Database Systems Using SQL](https://www.freecodecamp.org/news/how-to-design-structured-database-systems-using-sql-full-book/) -  Daniel García Solla, freeCodeCamp (HTML)
 * [Introduction to SQL](https://github.com/bobbyiliev/introduction-to-sql) - Bobby Iliev (Markdown, PDF)
 * [Learning SQL](https://riptutorial.com/Download/sql.pdf) - Compiled from StackOverflow Documentation (PDF)