KVM: PPC: Remove leftover comment from emulate_loadstore.c

Commit 4eeb8556

 ("KVM: PPC: Remove mmio_vsx_tx_sx_enabled in KVM
MMIO emulation") removed the mmio_vsx_tx_sx_enabled field, but its
documentation was left behind.  Remove the superfluous comment.

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>