[build] Fix build with HEAD clang.

The HEAD version of Clang fails to build boringssl due to the following
warning triggered at -Werror:

    /usr/local/google/home/dthorn/curl/boringssl/tool/digest.cc:229:12:
    error: variable 'bad_hash_lines' set but not used

This change removes the variable.

Change-Id: I25009925d9a2dfc5b1783accab19e4b861db4ec2
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/52265
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>

tool/digest.cc

@@ -226,7 +226,6 @@ static bool Check(const CheckModeArguments &args, const EVP_MD *md,
   unsigned bad_lines = 0;
   unsigned parsed_lines = 0;
   unsigned error_lines = 0;
-  unsigned bad_hash_lines = 0;
   unsigned line_no = 0;
   bool ok = true;
   bool draining_overlong_line = false;
@@ -297,7 +296,6 @@ static bool Check(const CheckModeArguments &args, const EVP_MD *md,
     }
 
     if (calculated_hex_digest != std::string(line, hex_size)) {
-      bad_hash_lines++;
       if (!args.status) {
         printf("%s: FAILED\n", target_filename.c_str());
       }