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
```

0 files changed, 0 insertions, 0 deletions