build-unit-test-docker: remove openbmc/linux dependency

Downloading the entire openbmc/linux repository is pretty slow
due to the way the Github CDN works on these kinds of files (and
how large it is).  On my home connection I typically see ~100kbps
on this download, which would equate to over 15 minutes of a download.

This was originally done so that we could get kernel headers for
interfaces that we didn't have upstreamed yet but everything is now
upstreamed so this is not necessary.

If this becomes necessary again in the future, it would be better
to explicitly download specific headers from the respository instead
of attempting to get the entire thing.

Tested:

The necessary headers are now provided by `linux-libc-dev`:
```
$ docker run --rm -it openbmc/ubuntu-unit-test:2023-W15-c977bb8f48b9ee3b
root@e20efded42fb:/# dpkg -S /usr/include/linux/ipmi_bmc.h
linux-libc-dev:amd64: /usr/include/linux/ipmi_bmc.h
root@e20efded42fb:/# dpkg -S /usr/include/linux/bt-bmc.h
linux-libc-dev:amd64: /usr/include/linux/bt-bmc.h
```

Also manually ran `btbridge` and `ipmbbridge` under the
`run-unit-test-docker.sh` with successful results.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I7689c59e6cb290ac1a703cbf7994d2d9b6434482
diff --git a/scripts/build-unit-test-docker b/scripts/build-unit-test-docker
index 5cb4445..87a565d 100755
--- a/scripts/build-unit-test-docker
+++ b/scripts/build-unit-test-docker
@@ -155,15 +155,6 @@
         rev="v1.15",
         build_type="make",
     ),
-    # dev-6.0 2022-11-28
-    "openbmc/linux": PackageDef(
-        rev="1b16243b004ce4d977a9f3b9d9e715cf5028f867",
-        build_type="custom",
-        build_steps=[
-            f"make -j{proc_count} defconfig",
-            f"make INSTALL_HDR_PATH={prefix} headers_install",
-        ],
-    ),
     "LibVNC/libvncserver": PackageDef(
         rev="LibVNCServer-0.9.13",
         build_type="cmake",