bpf: Remove set but not used variable 'dst_known'

Fixes gcc '-Wunused-but-set-variable' warning:

kernel/bpf/verifier.c:5603:18: warning: variable ‘dst_known’
set but not used [-Wunused-but-set-variable], delete this
variable.

Signed-off-by: Mao Wenan <maowenan@huawei.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200418013735.67882-1-maowenan@huawei.com