linux_common: retire extra module version.

The second 'linuxcommon' line was added by c66f5b079d2a259c3a65b1efe0f2143cd030dc52
but Linuxulator's modules dependend on 'linux_common'.
To avoid such mistakes in the future rename moduledata name and module
name to 'linux_common' and retire 'linuxcommon' line.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D30409
MFC after: 2 weeks