[packman] Fix for ffmpeg-8

[Antonio Larrosa]

Hi, updating my system is giving me tons of problems that at the end are
related to this zypper message:

```
> LANG=C sudo zypper in  ffmpeg-8-8.1.2-1699.6.pm.35.x86_64
Loading repository data...
Reading installed packages...
Resolving package dependencies...

Problem: 1: the to be installed ffmpeg-8-8.1.2-1699.6.pm.35.x86_64 requires
'(libavcodec62 = 8.1.2-1699.6.pm.35 or ffmpeg-8-mini-libs =
8.1.2-1699.6.pm.35)', but this requirement cannot be provided
not installable providers:
libavcodec62-8.1.2-1699.6.pm.35.i586[ftp.gwdg.de-Essentials]

 libavcodec62-8.1.2-1699.6.pm.35.x86_64[ftp.gwdg.de-Essentials]

 libavcodec62-8.1.2-1699.6.pm.35.i586[ftp.gwdg.de-openSUSE_Tumbleweed]

 libavcodec62-8.1.2-1699.6.pm.35.x86_64[ftp.gwdg.de-openSUSE_Tumbleweed]

 Solution 1: do not install ffmpeg-8-8.1.2-1699.6.pm.35.x86_64
 Solution 2: break ffmpeg-8-8.1.2-1699.6.pm.35.x86_64 by ignoring some of
its dependencies

nothing provides 'this-is-only-for-build-envs' needed by the to be
installed ffmpeg-8-mini-libs-8.1.2-6.1.x86_64
```

After having a look at the packages in packman I found the problem is
caused by the _multibuild file being removed.

I don't have permissions to fix it so if someone with permissions can go to
https://pmbs.links2linux.de/package/show/Essentials/A_tw-ffmpeg-8?expand=1
, click on "show unmerged sources", then open the _link file and remove the
<patches> section (or at least, the <delete name="_multibuild"/> entry),
that would fix the build.

Actually, I tested on home:antlarr:branches:Essentials/A_tw-ffmpeg-8 that
removing that AND updating to the latest version from multimedia:libs fixes
the package (the latest changes that were submitted together with the
update to 8.1.3 are needed to fix an unrelated compilation issue).

About the "Weird usage of multibuild" comment in _link... the "mini" flavor
is really needed here since ffmpeg has some dependencies which actually
depend on ffmpeg, so we need a minimum version of ffmpeg to build in order
to build the dependencies that are used to build the full ffmpeg version.
Having that minimal ffmpeg version require a non-existant
`this-is-only-for-build-envs` capability makes sure that minimal version is
never installed on a user system.

Thanks!

-- 
Antonio Larrosa