Remove forgotten LinuxMint occurrence

Right now we don't officialy support LinuxMint as our
documentation says [1], it seems LinuxMint is a relict
and got forgotten over time.

This patch removes LinuxMint from the code in order not to
confuse users.

[1] https://docs.openstack.org/devstack/latest/
Closes-Bug: #1983427

Change-Id: Ie1ced25f89389494b28a7b2e9bb1c4273e002dd5
diff --git a/doc/source/plugins.rst b/doc/source/plugins.rst
index 7d70d74..62dd15b 100644
--- a/doc/source/plugins.rst
+++ b/doc/source/plugins.rst
@@ -238,7 +238,7 @@
 locations in the top-level of the plugin repository:
 
 - ``./devstack/files/debs/$plugin_name`` - Packages to install when running
-  on Ubuntu, Debian or Linux Mint.
+  on Ubuntu or Debian.
 
 - ``./devstack/files/rpms/$plugin_name`` - Packages to install when running
   on Red Hat, Fedora, or CentOS.