sh: stacktrace: Remove stacktrace_ops.stack()

The SH implementation never called stacktrace_ops.stack().
Presumably this was copied from the x86 implementation.

Hence remove the method, and all implementations (most of them are
dummies).

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rich Felker <dalias@libc.org>