fastboot: Remove "slot-suffixes" variable

"slot-suffixes" variable was dropped in fastboot tool (in [1]). Let's
track AOSP/master and drop this variable in U-Boot as well.

[1] https://android.googlesource.com/platform/system/core/+/42b18a518bac85c3eea14206f6cbafbd1e98a31f



Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
[lukma: Remove getvar_slot_suffixes() declaration]
Signed-off-by: Lukasz Majewski <lukma@denx.de>