powerpc/perf: remove current_is_64bit()

Since commit ed1cd6de

 ("powerpc: Activate CONFIG_THREAD_INFO_IN_TASK")
current_is_64bit() is quivalent to !is_32bit_task().
Remove the redundant function.

Suggested-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190912194633.12045-1-msuchanek@suse.de