tcmode-default: eliminate needless linux-libc-headers version defaults

tcmode-default: eliminate needless linux-libc-headers version defaults

Manually setting the preferred version is not required or generally a
good idea as stated in:
   meta/recipes-kernel/linux-libc-headers/linux-libc-headers.inc

   # You're probably looking here thinking you need to create some new copy
   # of linux-libc-headers since you have your own custom kernel. To put
   # this simply, you DO NOT.

so remove that for linux-libc-headers.

Signed-off-by: Randy MacLeod <Randy.MacLeod@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>