Use a pre-existing target for systemd services

Providing mail.target is really confusing and poor UX:

* When a user enables a unit, it's not truly enabled until they ALSO
  enable mail.target. This is very counter-intuitive.
* `mail.target` provides no extra value in itself, nor is it anything
  "standard".
* Any user wanting this specific target can still continue using it just
  dropping in a `mail.target` file.

Signed-off-by: Hugo Osvaldo Barrera <hugo@barrera.io>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

contrib/systemd/README.md

@@ -21,10 +21,6 @@ start offlineimap.service` or by enabling the timer unit using `systemctl --user
 enable offlineimap.timer`. Additionally, specific accounts may be triggered by
 using `offlineimap@myaccount.timer` or `offlineimap@myaccount.service`.
 
-These unit files are installed as being enabled via a `mail.target` unit which
-is intended to be a catch-all for mail-related unit files. A simple
-`mail.target` file is also provided.
-
 If the defaults provided by these units doesn't suit your setup, any of the
 values may be overridden by using `systemctl --user edit offlineimap.service`.
 This'll prevent having to copy-and-edit the original file.

contrib/systemd/mail.target

@@ -1,5 +0,0 @@
-[Unit]
-Description=Mail Target
-
-[Install]
-WantedBy=default.target

contrib/systemd/offlineimap-oneshot@.service

@@ -9,4 +9,4 @@ ExecStart=/usr/bin/offlineimap -o -a %i -u basic
 TimeoutStopSec=120
 
 [Install]
-WantedBy=mail.target
+WantedBy=default.target

contrib/systemd/offlineimap.service

@@ -8,4 +8,4 @@ Restart=on-failure
 RestartSec=60
 
 [Install]
-WantedBy=mail.target
+WantedBy=default.target

contrib/systemd/offlineimap.timer

@@ -7,4 +7,4 @@ OnUnitInactiveSec=15m
 Unit=offlineimap-oneshot.service
 
 [Install]
-WantedBy=mail.target
+WantedBy=default.target

contrib/systemd/offlineimap@.service

@@ -8,4 +8,4 @@ Restart=on-failure
 RestartSec=60
 
 [Install]
-WantedBy=mail.target
+WantedBy=default.target

contrib/systemd/offlineimap@.timer

@@ -7,4 +7,4 @@ OnUnitInactiveSec=15m
 Unit=offlineimap-oneshot@%i.service
 
 [Install]
-WantedBy=mail.target
+WantedBy=default.target