semaphore: remove the Semaphore repositories recursively

The list of disabled repositories was recently converted from a single
file into a directory with separate repository files, so let's adjust
the setup script accordingly.

```
$ ls -lR /etc/apt/sources.list.d/
/etc/apt/sources.list.d/:
total 36
-rw-r--r-- 1 root root   76 Nov  3 10:28 azure-cli.list
-rw-r--r-- 1 root root   72 Nov  3 10:22 bazel.list
drwxr-xr-x 2 root root 4096 Nov  3 10:31 disabled
-rw-r--r-- 1 root root  113 Nov  3 10:13 docker-source.list
-rw-r--r-- 1 root root  367 Nov  3 10:28 github_git-lfs.list
-rw-r--r-- 1 root root  111 Nov  3 10:25 google-chrome-source.list
-rw-r--r-- 1 root root   64 Nov  3 10:14 google-cloud-sdk.list
-rw-r--r-- 1 root root   54 Nov  3 10:23 helm-stable-debian.list
-rw-r--r-- 1 root root   89 Nov  3 10:29 yarn-source.list

/etc/apt/sources.list.d/disabled:
total 20
-rw-r--r-- 1 root root 100 Nov  3 10:23 devel_kubic_libcontainers_stable.list
-rw-r--r-- 1 root root 103 Nov  3 10:27 git.list
-rw-r--r-- 1 root root 105 Nov  3 10:22 gradle.list
-rw-r--r-- 1 root root 118 Nov  3 10:13 pypy.list
-rw-r--r-- 1 root root 104 Nov  3 10:13 python.list
```

.semaphore/semaphore-runner.sh

@@ -55,7 +55,7 @@ for phase in "${PHASES[@]}"; do
     case "$phase" in
         SETUP)
             # remove semaphore repos, some of them don't work and cause error messages
-            sudo rm -f /etc/apt/sources.list.d/*
+            sudo rm -rf /etc/apt/sources.list.d/*
 
             # enable backports for latest LXC
             echo "deb http://archive.ubuntu.com/ubuntu $UBUNTU_RELEASE-backports main restricted universe multiverse" | sudo tee -a /etc/apt/sources.list.d/backports.list