[libc++] [string_view] Remove operators made redundant by C++20 (#66206)

Thanks to Giuseppe D'Angelo for pointing this out on the cpplang Slack!

The example implementation in https://eel.is/c++draft/string.view.comparison#example-1
was necessary when it was written, in C++17, but in C++20 we don't need that
complexity anymore, because of the reversed candidates that are
synthesized by the compiler.