x86: fix last commit

Keeping the selector that was loaded from the 32-bit GDT is okay, because
only code segment descriptors differ between 32- and 64-bit mode.  In
fact the same is true for %ss as well, so let's just remove the whole
segment loading from load_tss.

Thanks to Bandan Das for debugging.

Reported-by: Shih-Wei Li <shihwei@cs.columbia.edu>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>