riscv: remove unused variable in ftrace

Noticed while building kernel-4.20.0-0.rc5.git2.1.fc30 for
Fedora 30/RISCV.

[..]
BUILDSTDERR: arch/riscv/kernel/ftrace.c: In function 'prepare_ftrace_return':
BUILDSTDERR: arch/riscv/kernel/ftrace.c:135:6: warning: unused variable 'err' [-Wunused-variable]
BUILDSTDERR:   int err;
BUILDSTDERR:       ^~~
[..]

Signed-off-by: David Abdurachmanov <david.abdurachmanov@gmail.com>
Fixes: e949b6db ("riscv/function_graph: Simplify with function_graph_enter()")
Reviewed-by: Olof Johansson <olof@lixom.net>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>