samples: bpf: Fix build error

GCC 10 is very strict about symbol clash, and lwt_len_hist_user contains
a symbol which clashes with libbpf:

/usr/bin/ld: samples/bpf/lwt_len_hist_user.o:(.bss+0x0): multiple definition of `bpf_log_buf'; samples/bpf/bpf_load.o:(.bss+0x8c0): first defined here
collect2: error: ld returned 1 exit status

bpf_log_buf here seems to be a leftover, so removing it.
Signed-off-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200511113234.80722-1-mcroce@redhat.com

samples/bpf/lwt_len_hist_user.c

@@ -15,8 +15,6 @@
 #define MAX_INDEX 64
 #define MAX_STARS 38
 
-char bpf_log_buf[BPF_LOG_BUF_SIZE];
-
 static void stars(char *str, long val, long max, int width)
 {
 	int i;