Stop explicitly supported Fedora

It's a moving target that keeps breaking. Right now it's broken because
removing openssl-devel causes removal of dkms, which DIB insists on
installing.

Document the supported distributions based on what the CI runs
(yes, it's Ubuntu Bionic and openSUSE 15.1, there is no mistake).

Change-Id: I08263751ca158db5883d95407ad0f3c9807826eb

dib/ironic-python-agent-ramdisk/test-elements/build-succeeds-fedora/element-deps

@@ -1,2 +0,0 @@
-fedora-minimal
-openstack-ci-mirrors

dib/ironic-python-agent-ramdisk/test-elements/build-succeeds-fedora/element-type

@@ -1 +0,0 @@
-ramdisk

doc/source/admin/dib.rst

@@ -2,7 +2,16 @@ diskimage-builder images
 ========================
 
 Images built using diskimage-builder_ are recommended for production use on
-real hardware.
+real hardware. The recommended distributions are:
+
+* CentOS 8 (regular and stream)
+* Debian Buster (``debian-minimal`` element)
+
+The following should work but receive only limited testing and support:
+
+* CentOS 7 (using Python 3.6)
+* openSUSE Leap 15.1
+* Ubuntu Bionic
 
 Building
 --------
@@ -34,7 +43,7 @@ variable (default is amd64):
 .. code-block:: shell
 
     export ARCH=aarch64
-    ironic-python-agent-builder -o my-ipa fedora
+    ironic-python-agent-builder -o my-ipa --release 8-stream centos
 
 ... with diskimage-builder
 ~~~~~~~~~~~~~~~~~~~~~~~~~~

releasenotes/notes/no-fedora-aa65bcc43f9d56fa.yaml

@@ -0,0 +1,4 @@
+---
+deprecations:
+  - |
+    Fedora is no longer explicitly supported for DIB-based builds.

zuul.d/ironic-python-agent-builder-jobs.yaml

@@ -112,15 +112,6 @@
     vars:
       extra_elements: ["extra-hardware"]
 
-- job:
-    name: ironic-python-agent-check-image-dib-fedora
-    parent: ironic-python-agent-check-image-base
-    required-projects:
-      - openstack/diskimage-builder
-    vars:
-      image_type: 'dib'
-      image_distro: 'fedora'
-
 - job:
     name: ironic-python-agent-check-image-dib-opensuse
     parent: ironic-python-agent-check-image-base

zuul.d/project.yaml

@@ -17,8 +17,6 @@
         # Non-voting jobs
         - ironic-python-agent-check-image-dib-centos7-python3:
             voting: false
-        - ironic-python-agent-check-image-dib-fedora:
-            voting: false
         - ironic-python-agent-check-image-dib-ubuntu:
             voting: false
         - ironic-python-agent-check-image-dib-opensuse: