toolchain: gcc: Remove support for GCC 5

toolchain: gcc: Remove support for GCC 5

GCC was used in 17.01 as the default compiler the last time. We do not
test this old GCC version any more and there are some known problems it
fails to compile the U-Boot for the Allwinner A64 SoC.

Just remove it to make it clear that we will not support this old GCC
version any more.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Paul Spooren <mail@aparcar.org>