Remove Solaris 10 from the list of supported platforms

Change 5023 (present in BIND 9.13.3+) removed BIND's internal implementation of the getifaddrs() function which was required for iterating network interfaces on Solaris 10 as that system does not support that function natively. This change makes it impossible to compile BIND on Solaris 10, so the latter should no longer be listed in PLATFORMS.md as the description of the section it belongs to begins with:

The following are platforms on which BIND is known to build and run (...)