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: #2253

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>

src/doc/rust.md

@@ -112,8 +112,7 @@ production. See [tokens](#tokens) for more information.
 
 ## Input format
 
-Rust input is interpreted as a sequence of Unicode codepoints encoded in UTF-8,
-normalized to Unicode normalization form NFKC.
+Rust input is interpreted as a sequence of Unicode codepoints encoded in UTF-8.
 Most Rust grammar rules are defined in terms of printable ASCII-range codepoints,
 but a small number are defined in terms of Unicode properties or explicit
 codepoint lists. [^inputformat]