DEVELOPERS: drop entry related to inexisting directory
Commit
4e7dfe20bb8cf0cccfbd177321eff73e82c8d940 ("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>