selftests: net: tls: remove unused variable and code

When building selftests/net with clang, the compiler warn about the
function abs() see below:

tls.c:657:15: warning: variable 'len_compared' set but not used [-Wunused-but-set-variable]
        unsigned int len_compared = 0;
                     ^

Rework to remove the unused variable and the for-loop where the variable
'len_compared' was assinged.

Fixes: 7f657d5b ("selftests: tls: add selftests for TLS sockets")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>