powerpc/mm: Remove long disabled SLB code

We have a bunch of SLB related code in the tree which is there to handle
dynamic VSIDs - but currently it's all disabled at compile time. The
comments say "Keep that around for when we re-implement dynamic VSIDs".

But that was over 10 years ago (commit 3c726f8dee ("[PATCH] ppc64:
support 64k pages")). The chance that it would still work unchanged is
minimal, and in the meantime it's confusing to folks browsing/grepping
the code. If we ever want to re-instate it, it's in the git history.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Balbir Singh <bsingharora@gmail.com>

arch/powerpc/mm/slb.c

@@ -32,7 +32,6 @@ enum slb_index {
 };
 
 extern void slb_allocate_realmode(unsigned long ea);
-extern void slb_allocate_user(unsigned long ea);
 
 static void slb_allocate(unsigned long ea)
 {