clang-tidy: check readability-redundant-preprocessor, fix fallouts

Also:
- cipher_suite: merge `USE_MBEDTLS` `#if` blocks.

Ref: https://clang.llvm.org/extra/clang-tidy/checks/readability/redundant-preprocessor.html

Closes #20628

src/tool_help.c

@@ -281,13 +281,11 @@ void tool_help(const char *category)
         curl_msnprintf(cmdbuf, sizeof(cmdbuf), "\n    --no-%s", a->lname);
       else
         curl_msnprintf(cmdbuf, sizeof(cmdbuf), "\n    %s", category);
-#ifdef USE_MANUAL
       if(a->cmd == C_XATTR)
         /* this is the last option, which then ends when FILES starts */
         showhelp("\nALL OPTIONS\n", cmdbuf, "\nFILES");
       else
         showhelp("\nALL OPTIONS\n", cmdbuf, "\n    -");
-#endif
     }
 #else
     curl_mfprintf(tool_stderr, "Cannot comply. "