DEVELOPERS: drop entry related to inexisting directory

Commit
4e7dfe20 ("configs/friendlyarm_nanopi_m4:
remove defconfig") forgot to fully reflect the removal of the
defconfig in the DEVELOPERS file, causing a get-developers warning:

WARNING: 'board/friendlyarm/nanopi-m4' doesn't match any file

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>