powerpc/irq: drop __irq_offset_value

This patch drops__irq_offset_value which has not been used since
commit 9c4cb825 ("powerpc: Remove use of CONFIG_PPC_MERGE")

This removes a sparse warning.

Fixes: 9c4cb825 ("powerpc: Remove use of CONFIG_PPC_MERGE")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/irq.c

@@ -81,10 +81,7 @@
 DEFINE_PER_CPU_SHARED_ALIGNED(irq_cpustat_t, irq_stat);
 EXPORT_PER_CPU_SYMBOL(irq_stat);
 
-int __irq_offset_value;
-
 #ifdef CONFIG_PPC32
-EXPORT_SYMBOL(__irq_offset_value);
 atomic_t ppc_n_lost_interrupts;
 
 #ifdef CONFIG_TAU_INT