configs/firefly_rk3288: remove defconfig

configs/firefly_rk3288: remove defconfig

The original submitter of this defconfig:

"""
Sorry, unfortunately we no longer have the firefly board,
so we can't maintain it.
"""

And this defconfig fails to build with gcc 6.x, causing breakage in
the defconfig testing.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/55306815
  https://gitlab.com/buildroot.org/buildroot/-/jobs/55306814

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit b0fd706c2fe77157ec9d157568acf535d7d8daca)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>