Attempting to compile security/sandbox with -std=c++20 hits this build error:
> security/sandbox/chromium/base/strings/string_util.cc:1043:53: error: overload resolution selected deleted operator '<<'
> | DLOG(ERROR) << "Invalid placeholder: $" << *i;
This error is caused because C++20 removed basic_string's `operator<<` overloads for char types because the overloads didn't behave as callers expected: instead of printing the character value ('A'), they treated the char as an integer value (65):
https://en.cppreference.com/w/cpp/io/basic_ostream/operator_ltlt2
mozilla-central's security/sandbox code doesn't actually use these `ReplaceStringPlaceholders` functions, so just remove them instead of cherry picking Chromium's upstream fix.
Differential Revision: https://phabricator.services.mozilla.com/D201675
