patch 9.1.0297: Patch 9.1.0296 causes too many issues

Problem:  Patch 9.1.0296 causes too many issues
          (Tony Mechelynck, @chdiza, CI)
Solution: Back out the change for now

Revert "patch 9.1.0296: regexp: engines do not handle case-folding well"

This reverts commit 7a27c108e0509f3255ebdcb6558e896c223e4d23 it causes
issues with syntax highlighting and breaks the FreeBSD and MacOS CI. It
needs more work.

fixes: #14487

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/regexp.c b/src/regexp.c
index 4e85ebc..4373ae0 100644
--- a/src/regexp.c
+++ b/src/regexp.c
@@ -1606,9 +1606,7 @@
 /*
  * Compare two strings, ignore case if rex.reg_ic set.
  * Return 0 if strings match, non-zero otherwise.
- * Correct the length "*n" when composing characters are ignored
- * or for utf8 when both utf codepoints are considered equal because of
- * case-folding but have different length (e.g. 's' and 'ſ')
+ * Correct the length "*n" when composing characters are ignored.
  */
     static int
 cstrncmp(char_u *s1, char_u *s2, int *n)
@@ -1617,13 +1615,6 @@
 
     if (!rex.reg_ic)
 	result = STRNCMP(s1, s2, *n);
-    else if (enc_utf8)
-    {
-	int l2 = mb_ptr2len(s2);
-	result = MB_STRNICMP2(s1, s2, *n, l2);
-	if (result == 0 && l2 < *n)
-	    *n = l2;
-    }
     else
 	result = MB_STRNICMP(s1, s2, *n);