patch 8.1.2120: some MB_ macros are more complicated than necessary

Problem:    Some MB_ macros are more complicated than necessary. (Dominique
            Pelle)
Solution:   Simplify the macros.  Expand inline.
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 26d8691..4a04696 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -6454,7 +6454,7 @@
 	    }
 	    else
 		set_last_csearch(PTR2CHAR(csearch),
-						csearch, MB_PTR2LEN(csearch));
+						csearch, mb_ptr2len(csearch));
 	}
 
 	di = dict_find(d, (char_u *)"forward", -1);