toolchain/gdb: get rid of legacy & unused variables

The BR2_VENDOR_SUFFIX and VENDOR_SUFFIX variables are no longer settable.
The only user is gdb, and is totally useless in this case.

Signed-off-by: Yann E. MORIN <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>