feat: Remove obs-home-deltachat.gpg

- We don't install Dovecot from OBS anymore.
- Removed files.put() that creates
  /etc/apt/keyrings/obs-home-deltachat.gpg; replaced this with a
  files.file() that sets present=False to remove the file from any
  existing installations where it already has been installed.
- Removed now-unused obs-home-deltachat.gpg file.
- Clarified description of sources.list operation.
- Suggested in review by missytake and hpk42.

cmdeploy/src/cmdeploy/__init__.py

@@ -1055,20 +1055,11 @@ class ChatmailDeployer(Deployer):
 
     @staticmethod
     def install_impl():
-        # Add our OBS repository for dovecot_no_delay
-        files.put(
-            name="Add Deltachat OBS GPG key to apt keyring",
-            src=importlib.resources.files(__package__).joinpath(
-                "obs-home-deltachat.gpg"
-            ),
-            dest="/etc/apt/keyrings/obs-home-deltachat.gpg",
-            user="root",
-            group="root",
-            mode="644",
-        )
+        # Remove OBS repository key that is no longer used.
+        files.file("/etc/apt/keyrings/obs-home-deltachat.gpg", present=False)
 
         files.line(
-            name="Add DeltaChat OBS home repository to sources.list",
+            name="Remove DeltaChat OBS home repository from sources.list",
             path="/etc/apt/sources.list",
             line="deb [signed-by=/etc/apt/keyrings/obs-home-deltachat.gpg] https://download.opensuse.org/repositories/home:/deltachat/Debian_12/ ./",
             escape_regex_characters=True,