patch 8.1.0812: Unicode 16 feature is not useful

Problem:    Unicode 16 feature is not useful and cannot be detected.
Solution:   Remove UNICODE16.
diff --git a/src/screen.c b/src/screen.c
index 95a739a..a80e8de 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -2515,12 +2515,7 @@
 			prev_c = u8c;
 #endif
 		    /* Non-BMP character: display as ? or fullwidth ?. */
-#ifdef UNICODE16
-		    if (u8c >= 0x10000)
-			ScreenLinesUC[idx] = (cells == 2) ? 0xff1f : (int)'?';
-		    else
-#endif
-			ScreenLinesUC[idx] = u8c;
+		    ScreenLinesUC[idx] = u8c;
 		    for (i = 0; i < Screen_mco; ++i)
 		    {
 			ScreenLinesC[i][idx] = u8cc[i];
@@ -4482,34 +4477,17 @@
 
 		    if ((mb_l == 1 && c >= 0x80)
 			    || (mb_l >= 1 && mb_c == 0)
-			    || (mb_l > 1 && (!vim_isprintc(mb_c)
-# ifdef UNICODE16
-							 || mb_c >= 0x10000
-# endif
-							 )))
+			    || (mb_l > 1 && (!vim_isprintc(mb_c))))
 		    {
 			/*
 			 * Illegal UTF-8 byte: display as <xx>.
 			 * Non-BMP character : display as ? or fullwidth ?.
 			 */
-# ifdef UNICODE16
-			if (mb_c < 0x10000)
-# endif
-			{
-			    transchar_hex(extra, mb_c);
+			transchar_hex(extra, mb_c);
 # ifdef FEAT_RIGHTLEFT
-			    if (wp->w_p_rl)		/* reverse */
-				rl_mirror(extra);
+			if (wp->w_p_rl)		/* reverse */
+			    rl_mirror(extra);
 # endif
-			}
-# ifdef UNICODE16
-			else if (utf_char2cells(mb_c) != 2)
-			    STRCPY(extra, "?");
-			else
-			    /* 0xff1f in UTF-8: full-width '?' */
-			    STRCPY(extra, "\357\274\237");
-# endif
-
 			p_extra = extra;
 			c = *p_extra;
 			mb_c = mb_ptr2char_adv(&p_extra);
@@ -7478,15 +7456,6 @@
 		else
 		    u8c = utfc_ptr2char(ptr, u8cc);
 		mbyte_cells = utf_char2cells(u8c);
-#ifdef UNICODE16
-		/* Non-BMP character: display as ? or fullwidth ?. */
-		if (u8c >= 0x10000)
-		{
-		    u8c = (mbyte_cells == 2) ? 0xff1f : (int)'?';
-		    if (attr == 0)
-			attr = HL_ATTR(HLF_8);
-		}
-#endif
 #ifdef FEAT_ARABIC
 		if (p_arshape && !p_tbidi && ARABIC_CHAR(u8c))
 		{