Delete .travis.yml

.github/workflows/.travis.yml

-# R for travis: see documentation at https://docs.travis-ci.com/user/languages/r
-
-language: R
-repos:
-  CRAN: https://cloud.r-project.org
-cache: packages
-latex: false
-pandoc: false
-sudo: false
-install:
-  - Rscript -e "if(!requireNamespace('devtools')) install.packages('devtools')"
-  - Rscript -e "if(!requireNamespace('lucode')) devtools::install_github('pik-piam/lucode')"
-script:
-  - Rscript -e "null <- lucode::codeCheck(strict=TRUE)"