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>