patch 8.2.3636: Coverity warns for unreachable code

Problem:    Coverity warns for unreachable code.
Solution:   Remove unreachable else block.
diff --git a/src/gui_gtk_x11.c b/src/gui_gtk_x11.c
index 40d7712..4e89c4e 100644
--- a/src/gui_gtk_x11.c
+++ b/src/gui_gtk_x11.c
@@ -5604,7 +5604,7 @@
 		{
 		    slen++; // ligature char by char
 		}
-		else if (is_utf8)
+		else // is_utf8
 		{
 		    if ((*(cs + slen) & 0xC0) == 0x80)
 		    {
@@ -5638,10 +5638,6 @@
 			slen++;
 		    }
 		}
-		else
-		{
-		    slen++;
-		}
 	    }
 	    else
 	    {