Remove unused macros in -Wimplicit-function-declaration error with gcc 14.

```
In file included from fns.h:3,
                 from n1.c:10:
n1.c: In function 'get1ch':
mbwc.h:2:16: error: implicit declaration of function 'p9mbtowc'; did you mean 'mbtowc'? [-Wimplicit-function-declaration]
    2 | #define mbtowc p9mbtowc
      |                ^~~~~~~~
n1.c:752:26: note: in expansion of macro 'mbtowc'
  752 |                 if ((n = mbtowc(&wc, buf, p-buf)) >= 0)
      |                          ^~~~~~
```

--- 9base-6-origin/troff/fns.h
+++ 9base-6/troff/fns.h
@@ -1,7 +1,5 @@
 #define getline p9getline
 
-#include "mbwc.h"
-
 /*
  * other
  */
--- 9base-6-origin/troff/mbwc.c
+++ 9base-6/troff/mbwc.c
@@ -1,5 +1,4 @@
 #include <stdlib.h>
-#include "mbwc.h"
 
 /*
  * Use the FSS-UTF transformation proposed by posix.
--- 9base-6-origin/troff/mbwc.h
+++ /dev/null
@@ -1,5 +0,0 @@
-#define mblen p9mblen
-#define mbtowc p9mbtowc
-#define mbstowcs p9mbstowcs
-#define wctomb p9wctomb
-#define wcstombs p9wcstombs