powerpc: Remove in_kernel_text()

Last user of in_kernel_text() stopped using in with
commit 549e8152 ("powerpc: Make the 64-bit kernel as a
position-independent executable").

Generic function is_kernel_text() does the same.

So remote it.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/2a3a5b6f8cc0ef4e854d7b764f66aa8d2ee270d2.1624813698.git.christophe.leroy@csgroup.eu