powerpc/time: Remove get_tbu()

get_tbu() is redundant with mftbu() and is not used anymore.

Remove 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/1746e2d11ea90c3f45877e1fcc6c79ce96cf6b98.1601556145.git.christophe.leroy@csgroup.eu