fastboot: Remove FIXME for CONFIG_FASTBOOT_...NAME

CONFIG_FASTBOOT_GPT_NAME and CONFIG_FASTBOOT_MBR_NAME are always defined
by Kconfig if you're compiling this code, so remove these redundant
defaults.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>