debug/gdb: remove insight

Insight seems to be very slow to follow up on mainstreram gdb.
Latest snapshots are more than 6 months old.

Moreover, I don't have time to maintain insight support in crosstool-NG;
and, because I don't use it, I am unable to find any breakage.

reporter_name="Harold Grovesteen"

reporter_url="http://sourceware.org/ml/crossgcc/2009-11/msg00052.html"

reporter_comment=""