es.po: stop translating "kB" to "KiB", etc.

"kB" is used by GLib to mean 1000 bytes exactly -- not 1024.  It is
therefore incorrect to translate this to KiB.  Same for the other units.

The original commit ef3e5917 fixing this
problem was reverted, so I'm applying it again.

See http://mail.gnome.org/archives/gnome-i18n/2011-July/msg00078.html
for more information.