powerpc/ftrace: Simplify function_graph support in ftrace.c

Since we now support DYNAMIC_FTRACE_WITH_ARGS across ppc32 and ppc64
ELFv2, we can simplify function_graph tracer support code in ftrace.c

Signed-off-by: Naveen N Rao <naveen@kernel.org>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/4dc92c4b1ed444dc62b748ae7327acdb9e096864.1687166935.git.naveen@kernel.org