configure, meson: remove CONFIG_SOLARIS from config-host.mak

CONFIG_SOLARIS is only used to pick tap implementations.  But the
target OS is invariant and does not depend on the configuration, so move
away from config_host and just use unconditional rules in softmmu_ss.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>