Remove misleading comment for $wgLegalTitleChars

Remove misleading comment for $wgLegalTitleChars

This comment originates from rSVN1420 (9d51f616), dated 2 July 2003,
where it was written as "ISO 8859-* don't allow 0x80-0x9F... But that
breaks interlanguage links at the moment". It was rephrased to the
current form in rSVN2621 (840dee3a).

It is incorrect for two reasons:

* "Theoretically 0x80-0x9F of ISO 8859-1 should be disallowed..."

  We cannot disallow 0x80-0x9F here; this config variable actually
  specifies the valid ranges of *bytes* rather than characters, and
  0x80 to 0x9F can happily appear in valid UTF-8 encodings of other
  characters.

  In case we wanted to disallow the Unicode characters U+0080 to U+009F
  (encoded in UTF-8 as 0xC2 0x80 to 0xC2 0x9F), it would probably have
  to be done explicitly in MediaWikiTitleCodec::splitTitleString().
  (The task for this is T7732.)

* "...but this breaks interlanguage links"

  Back then, most wikis were using single-byte ISO encodings rather
  than UTF-8, and that is the only configuration this comment applies
  to: disallowing the bytes 0x80-0x9F in page titles on wikis using
  single-byte ISO encodings would indeed have broken interlanguage
  links from them to wikis using UTF-8. However, disallowing the
  Unicode characters U+0080 to U+009F today definitely would not break
  interlanguage links.

Change-Id: Ic5ba502ccfbb9cf3ff56cc47eb7fe463e7d45959