Remove the authentication cookie file

[eitsupi]

Fixes #137 #165

$ docker run --rm -it rocker/rstudio:4.1.0 ls -l /var/lib/rstudio-server/
total 36
drwxr-xr-x 2 root           root            4096 May 23 21:57 body
drwxr-xr-x 2 root           root            4096 May 23 21:57 conf
drwxr-xr-x 2 root           root            4096 May 23 21:57 proxy
-rw-r--r-- 1 rstudio-server rstudio-server 20480 May 23 21:57 rstudio.sqlite
-rw------- 1 root           root              32 May 23 21:57 secure-cookie-key

[cboettig]

thanks!