d/control: drop Build-Conflicts on gdb-minimal

d/control: drop Build-Conflicts on gdb-minimal

used to be needed to ensure the full gdb is used to satisfy the
build-dependency, since gdb-minimal also provided gdb. now (since gdb 15) it's
rwversed - gdb provides gdb-minimal, but since it also conflicts and replaces
it, a dependency on gdb, and a conflict on gdb-minimal are not satisfiable
anymore.

Signed-off-by: Fabian Grünbichler <git@fabian.gruenbichler.email>