selftests/bpf: Removed redundant fd after close in bpf_prog_load_log_buf

Removed unnecessary `fd = -1` assignments after closing file descriptors.
because it will be assigned by the function bpf_prog_load().This improves
code readability and removes redundant operations.

Signed-off-by: Zhu Jun <zhujun2@cmss.chinamobile.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20241010055737.4292-1-zhujun2@cmss.chinamobile.com