Revert "Fix cursor move with wide glyphs"

This reverts commit 7473a8d1a5.

This patch needs some more work. It caused regressions with programs that use
GNU readline, etc.

Original test-case example from Tim Culverhouse <tim@timculverhouse.com>:

	printf " 😀" && sleep 2 && printf "\e[D" && sleep 2 && printf "\e[D" && sleep 2

After the patch it caused regressions, example test-case:

	printf "A字\bB\n"

st.c

@@ -86,8 +86,8 @@ enum escape_state {
 
 typedef struct {
 	Glyph attr; /* current char attributes */
-	int x; /* terminal column */
-	int y; /* terminal row */
+	int x;
+	int y;
 	char state;
 } TCursor;
 
@@ -2175,16 +2175,12 @@ tstrsequence(uchar c)
 void
 tcontrolcode(uchar ascii)
 {
-	size_t i;
-
 	switch (ascii) {
 	case '\t':   /* HT */
 		tputtab(1);
 		return;
 	case '\b':   /* BS */
-		for (i = 1; term.c.x && term.line[term.c.y][term.c.x - i].u == 0; ++i)
-			;
-		tmoveto(term.c.x - i, term.c.y);
+		tmoveto(term.c.x-1, term.c.y);
 		return;
 	case '\r':   /* CR */
 		tmoveto(0, term.c.y);