refactor: delete duplicate utf8-functionality

Also remove British National Replacement Character Set.

We keep the DEC Special Graphics and ASCII despite it not being unicode
as some old software such as calcurse still rely on this functionality.

References:
- https://github.com/neovim/neovim/pull/31934#discussion_r1911046426
- https://en.wikipedia.org/wiki/DEC_Special_Graphics
- https://vt100.net/docs/vt220-rm/chapter2.html#S2.4.3

test/unit/vterm_spec.lua

@@ -28,6 +28,7 @@ local bit = require('bit')
 --- @field parser_sos function
 --- @field parser_text function
 --- @field print_color function
+--- @field schar_get fun(any, any):integer
 --- @field screen_sb_clear function
 --- @field screen_sb_popline function
 --- @field screen_sb_pushline function
@@ -43,6 +44,8 @@ local bit = require('bit')
 --- @field state_setpenattr function
 --- @field state_settermprop function
 --- @field term_output function
+--- @field utf_ptr2char fun(any):integer
+--- @field utf_ptr2len fun(any):integer
 --- @field vterm_input_write function
 --- @field vterm_keyboard_end_paste function
 --- @field vterm_keyboard_key function
@@ -360,7 +363,7 @@ local function screen_cell(row, col, expected, screen)
   pos['row'] = row
   pos['col'] = col
 
-  local cell = t.ffi.new('VTermScreenCell')
+  local cell = t.ffi.new('VTermScreenCell') ---@type any
   vterm.vterm_screen_get_cell(screen, pos, cell)
 
   local buf = t.ffi.new('unsigned char[32]')
@@ -1705,12 +1708,6 @@ putglyph 1f3f4,200d,2620,fe0f 2 0,4]])
     push('#', vt)
     expect('putglyph 23 1 0,0')
 
-    -- Designate G0=UK
-    reset(state, nil)
-    push('\x1b(A', vt)
-    push('#', vt)
-    expect('putglyph a3 1 0,0')
-
     -- Designate G0=DEC drawing
     reset(state, nil)
     push('\x1b(0', vt)