os::is_headless_jre() is currently in the sources for all UNIX platforms (aix,linux,solaris etc.), this can be centralized in os_posix; additionally the check for libmawt in the coding seems to be for old pre-JDK9 JDKs can can be removed.

old summary name was : Posix platforms : move os::is_headless_jre check coding to os::Posix and remove test for libmawt