lib/cpp: Remove CONFIG_CPP_MAIN

With Zephyr now always using `int main(void)`, there's no longer any need
for this definition. The last remaining use which gated the declaration of
_posix_zephyr_main isn't necessary as adding that declaration
unconditionally is harmless.

Signed-off-by: Keith Packard <keithp@keithp.com>
4 files changed