target-i386: Remove setting tsc-frequency from x86_def_t

Setting tsc-frequency from x86_def_t is NOP because default tsc_khz
in x86_def_t is 0 and CPUX86State.tsc_khz is also initialized to 0
by default. So there is no need to overwrite tsc_khz with default 0
because field was already initialized to 0.

Custom tsc-frequency setting is not affected due to it being set
without using x86_def_t.

Field tsc_khz in x86_def_t becomes unused with this patch, so drop it
as well.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
