Document that -singlestep command line option is deprecated

Document that -singlestep command line option is deprecated

Document that the -singlestep command line option is now
deprecated, as it is replaced by either the TCG accelerator
property 'one-insn-per-tb' for system emulation or the new
'-one-insn-per-tb' option for usermode emulation, and remove
the only use of the deprecated syntax from a README.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20230417164041.684562-7-peter.maydell@linaro.org