gcc: remove option on SJLJ exceptions

This is a very advanced option, and it seems, according to
http://choices.cs.uiuc.edu/exceptions.pdf

 that SJLJ exceptions aren't
really interesting.

Users really interested by this can always use the
BR2_EXTRA_GCC_CONFIG_OPTIONS is they want.

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