[libc++][NFC] Remove unimplemented parts of chrono synopsis

Several parts in the `chrono` synopsis for C++20 are not yet
implemented. The current recommendation is that things are added to the
synopsis when implemented -- not beforehand. As such, remove the
not-yet-implemented parts to avoid confusion.

Reviewed By: ldionne, Quuxplusone, #libc

Differential Revision: https://reviews.llvm.org/D111922