powerpc: Relocatable system call no longer uses the LR

We had some code to restore the LR in the relocatable system call path
back when we used the LR to do an indirect branch.

Commit 6a404806

 ("powerpc: Avoid link stack corruption in MMU
on syscall entry path") changed this to use the CTR which is volatile
across system calls so does not need restoring.

Remove the stale comment and the restore of the LR.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>