Remove all external links from posts

You can do this by a SQL query, example:

UPDATE wp_posts
SET post_content = CONCAT(
    LEFT(post_content, LOCATE('<a', post_content) - 1), 
    SUBSTRING(post_content, LOCATE('</a>', post_content, LOCATE('<a', post_content) + 2) + 4))
WHERE post_content LIKE '%<a%';