Autoconf: Remove Solaris prototype headers. [skip appveyor]

Autoconf: Remove Solaris prototype headers. [skip appveyor]

os-sunos4.h is for SunOS 4 and os-solaris2.h is for Solaris 2, which
both were released about 30 years ago.  However, config.guess still
detects Solaris-like OSes as "solaris2", so os-solaris2.h could
actually be used in .devel builds on respective OSes.  However, that
made no difference because tcpdump _compiles_ without the header just
fine even on Solaris 9, which was released more than 20 years ago.