Unlist external books which are 404 (#335)

Author: cderv

R/external.txt

@@ -43,7 +43,6 @@ https://engineering-shiny.org/
 https://enriquegit.github.io/behavior-free/
 https://envirometrix.github.io/PredictiveSoilMapping/
 https://epirhandbook.com/
-https://est-mult.netlify.com/
 https://f0nzie.github.io/rtorch-minimal-book/
 https://fg2re.sellorm.com/
 https://geanders.github.io/navy_public_health/
@@ -79,10 +78,6 @@ https://m-clark.github.io/text-analysis-with-R/
 https://manual.nssurge.com/book/understanding-surge/cn/
 https://manual.nssurge.com/book/understanding-surge/en/
 https://mixomicsteam.github.io/mixOmics-Vignette/
-https://myrbookfr.netlify.app/
-https://myrbookfr.netlify.com/
-https://myrbooksp.netlify.app/
-https://myrbooksp.netlify.com/
 https://nacnudus.github.io/spreadsheet-munging-strategies/
 https://neonira.github.io/offensiveProgrammingBook_v1.2.2/
 https://orchid00.github.io/actions_sandbox/