txt_db: fix -Wunused-but-set-variable

The loop never uses the value of 'ln'.

Fixes this error with Clang 15:
```
crypto/txt_db/txt_db.c:24:10: error: variable 'ln' set but not used [-Werror,-Wunused-but-set-variable]
    long ln = 0;
         ^
1 error generated.
```

Signed-off-by: Sam James <sam@gentoo.org>

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>

(cherry picked from commit f9e8e2c)
Signed-off-by: Sam James <sam@gentoo.org>

(Merged from #19500)

crypto/txt_db/txt_db.c

@@ -21,7 +21,6 @@ TXT_DB *TXT_DB_read(BIO *in, int num)
 {
     TXT_DB *ret = NULL;
     int esc = 0;
-    long ln = 0;
     int i, add, n;
     int size = BUFSIZE;
     int offset = 0;
@@ -61,7 +60,6 @@ TXT_DB *TXT_DB_read(BIO *in, int num)
         }
         buf->data[offset] = '\0';
         BIO_gets(in, &(buf->data[offset]), size - offset);
-        ln++;
         if (buf->data[offset] == '\0')
             break;
         if ((offset == 0) && (buf->data[0] == '#'))