toolchain/gdb: get rid of another legacy and unused variable

BR2_VENDOR_GDB_VERSION and VENDOR_GDB_VERSION are no longer settable.
The only user is gdb, and it's 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>