libgc: Remove duplicated package

Remove libgc that is a copy of bdwgc but it's not used
by any package and gives an error when building make-4.1.
bdwgc is a mandatory dependency for guile.

Fixes:
http://autobuild.buildroot.net/results/b80a34a602b1241bc03cd43df1a269389d50e75c/



Reported-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Pedro Aguilar <paguilar@paguilar.org>
[baruch: added Reported-by, Config.in.legacy entry, autobuilder reference]
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>