Remove now-redundant Tar::Solaris from module_tool

Changes to the way Puppet::ModuleTool::Tar::Gnu works means that the
*::Tar::Solaris variant should now be redundant. It existed to require
gtar when the underlying platform did not support GNU-tar options, but
the changes to the core Tar::Gnu class mean that we no longer use any
flags or options that don't work just fine with oldschool unix tar
executables. Remove Puppet::ModuleTool::Tar::Solaris.

On a side note, Tar::Gnu should probably be renamed to something more
generic.