posix: remove unneeded option to link with posix subsys

The CONFIG_APP_LINK_WITH_POSIX_SUBSYS option was originally
present so that internal POSIX implementation headers would be
on the include path.

There is no implicit need for any app or library to include
private POSIX headers. Instead, the standard POSIX API should
be used.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>