[OE-core] [PATCH] siteinfo: nios2-linux - remove wrong mutex info

[Bystricky, Juro]

> While I understand ARM/ is wrong, why is removing the db_cv_mutex a
> correct fix instead of ie. s/ARM/nios2/ ? The patch description does not
> explain that.

With the relatively recent commit http://git.yoctoproject.org/cgit.cgi/poky/commit/meta/recipes-support/db?id=f4e8e9242c5df095b52688b083df3c9d36e7144b
some nios2 builds (uboot, core-image-minimal, etc) were broken due to db trying to
use ARM instructions in mutexes. The reason was db "configure" used the cached entry 
from nios2-linux (which was incorrect).  So the remedy was to remove the incorrect cached entry
and let db "configure" figure out which is the proper mutex to use. 
Besides, I don't even know if nios2/gcc-assembly exists.
Also, none of other Linux info files provides this entry (db_cv_mutex).