refactor: remove special handling for lowercase German sharp s

utf8proc already defines LATIN CAPITAL LETTER SHARP S (ẞ) to be the
uppercase variant of LATIN SMALL LETTER SHARP S (ß), so this special
workaround when using `gU` is no longer needed on the neovim side.

src/nvim/mbyte.c

@@ -1326,9 +1326,6 @@ int utf_fold(int a)
 // invalid values or can't handle latin1 when the locale is C.
 // Speed is most important here.
 
-// Note: UnicodeData.txt does not define U+1E9E as being the corresponding upper
-// case letter for U+00DF (ß), however it is part of the toLower table
-
 /// Return the upper-case equivalent of "a", which is a UCS-4 character.  Use
 /// simple case folding.
 int mb_toupper(int a)