DEVELOPERS: remove Sebastien Bourdelin

DEVELOPERS: remove Sebastien Bourdelin

His e-mail address is no longer valid. Sending an e-mail to this
address says (yes, the actual text is in French):

Veuillez prendre note que cette adresse n'est plus valide à la suite du départ de l'employé.

which means:

Please take note that this address is no longer valid following the employee departure.

Therefore, it does not make sense to list Sebastien in our DEVELOPERS
file.

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