Remove the "NFKC clause" in the reference manual.

The reference manual said that code is interpreted as UTF-8 text and a implementation will normalize it to NFKC. However, rustc doesn't do any normalization now.

We may want to do any normalization for symbols, but normalizing whole text seems harmful because doing so loses some sort of information even if we choose a non-K variant of normalization.

I'd suggest removing "normalized to Unicode normalization form NFKC" phrase for the present so that the manual represents the current state properly. When we address the problem (with a RFC?), then the manual
should be updated.

Closes #12388.

Reference: https://github.com/rust-lang/rust/issues/2253