netfilter: xt_TPROXY: remove pr_debug invocations

pr_debug calls are no longer needed in this file.

Pablo suggested "a patch to remove these pr_debug calls". This patch has
some other beneficial collateral as it also silences multiple Clang
-Wformat warnings that were present in the pr_debug calls.

diff from v1 -> v2:
* converted if statement one-liner style
* x == NULL is now !x

Suggested-by: Pablo Neira Ayuso <pablo@netfilter.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>