Contra documentation, now() requires std

[dfoxfranke]

The chrono documentation describes the clock feature as follows:

clock: enables reading the system time (now), independent of whether std::time::SystemTime is present, depends on having a libc.

But this is not the case: clock pulls in std, and the implementation of Utc::now() is just a wrapper around std::time::SystemTime::now().

[pitdicker]

The current readme describes the clock feature as:

• clock: Enables reading the system time (now) and local timezone (Local).

And the main documentation in lib.rs:

• clock: Enables reading the system time (now) that depends on the standard library for UNIX-like operating systems and the Windows API (winapi) for Windows.

The documentation in lib.rs still needs some updating, but at least we no longer it is "independent of whether std::time::SystemTime is present".

[pitdicker]

The documentation in lib.rs is updated in #1260.