release: remove Oracle Cloud Infrastructure build targets

[dch]

Summary

Remove all the Oracle flavoured release tooling bits from the tree.
I intend to MFC this in time for 2026-04-13's next stable/15 build cycle.

Relnotes: yes
Sponsored by: SkunkWerks, GmbH
MFC after: 3 days

Test Plan

set -x SRCCONF /dev/null
set -x SRC_ENV_CONF /dev/null
doas make -j2C buildworld  TARGET_ARCH=amd64 TARGET=amd64 -s \
&& doas make -j2C buildkernel TARGET_ARCH=amd64 TARGET=amd64 KERNCONF=GENERIC -s \
&& cd ./release \
&& doas make -j2C clean \
&& doas make  KERNCONF=GENERIC TARGET_ARCH=amd64 TARGET=amd64 \
  WITH_CLOUDWARE=yes \
  CLOUDWARE=OCI \
  -s clean cloudware-release

Works on My Machine(tm)

[ivy]

the commit message should probably indicate why this is being removed...

[cperciva]

LGTM, but as @ivy noted the commit message should provide some of the context for why we're dropping Oracle images. (If nothing else, because it might provoke someone at Oracle to fix the situation.)