Fix unused-but-set-variable warnings by llvm-15

[masaori335]

llvm-15 (clang-15) generates below "unused-but-set-variable" warnings.

Cache.cc:2892:11: warning: variable 'disk_full' set but not used [-Wunused-but-set-variable]
      int disk_full         = 0;
          ^      
Store.cc:306:7: warning: variable 'ln' set but not used [-Wunused-but-set-variable]
  int ln          = 0;
      ^
RemapConfig.cc:151:17: warning: variable 'idx' set but not used [-Wunused-but-set-variable]
  for (unsigned idx = 1; str; idx++) {
                ^
LogObject.cc:1131:12: warning: variable 'num_kept_objects' set but not used [-Wunused-but-set-variable]
  unsigned num_kept_objects = 0;      
           ^
traffic_cache_tool/CacheDefs.cc:545:9: warning: variable 'len' set but not used [-Wunused-but-set-variable]
    int len          = 0;
        ^
gold_tests/tls/ssl-post.c:108:7: warning: variable 'read_count' set but not used [-Wunused-but-set-variable]
  int read_count     = 0;
      ^
gold_tests/tls/ssl-post.c:109:7: warning: variable 'write_count' set but not used [-Wunused-but-set-variable]
  int write_count    = 1;
      ^

The release note of clang 15.0.0 says below.

Improvements to Clang’s diagnostics
...
  * -Wunused-but-set-variable now also warns if the variable is only used by unary operators.
...  

https://releases.llvm.org/15.0.0/tools/clang/docs/ReleaseNotes.html#improvements-to-clang-s-diagnostics

[bneradt]

Thank you for helping keep the latest compilers happy.

Hopefully CI will help keep these clean in a month or so once fedora:37 is released and we update our github Fedora build to that. Apparently it should have llvm 15 as default:
https://fedoraproject.org/wiki/Releases/37/ChangeSet#LLVM_15

[zwoop]

Cherry-picked to v9.2.x