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)

src/nvim/ops.c

@@ -2148,18 +2148,6 @@ bool swapchar(int op_type, pos_T *pos)
  return false;
  }
 
- // ~ is OP_NOP, g~ is OP_TILDE, gU is OP_UPPER
- if ((op_type == OP_UPPER || op_type == OP_NOP || op_type == OP_TILDE) && c == 0xdf) {
- pos_T sp = curwin->w_cursor;
-
- // Special handling for lowercase German sharp s (ß): convert to uppercase (ẞ).
- curwin->w_cursor = *pos;
- del_char(false);
- ins_char(0x1E9E);
- curwin->w_cursor = sp;
- return true;
- }
-
  int nc = c;
  if (mb_islower(c)) {
  if (op_type == OP_ROT13) {