linux: Remove obsolete version 4.7.2-97

Linux 4.7.2-97 is superseded by Linux 4.7.10, and Linux 4.8.4. Also, it
has the “[Dirty Cow](https://dirtycow.ninja/)” vulnerability. So remove
it.

```
$ pssh -i -H "`hostconfig --list mx64`" uname -a | grep "4.7"
Linux cu.molgen.mpg.de 4.7.2.mx64.99 #1 SMP Tue Aug 23 10:10:21 CEST 2016 x86_64 GNU/Linux
Linux vitricophobie.molgen.mpg.de 4.7.2.mx64.99 #1 SMP Tue Aug 23 10:10:21 CEST 2016 x86_64 GNU/Linux
```