Fixing trivial warnings spotted by Mingw-2.0.0/gcc-3.2:

plink.c: In function `from_backend':
  plink.c:172: warning: unused variable `h'
(introduced in 1.35 [r1198])

[originally from svn r2331]
[r1198 == c87fa98d09]

plink.c

@@ -169,7 +169,6 @@ void try_output(int is_stderr)
 
 int from_backend(void *frontend_handle, int is_stderr, char *data, int len)
 {
-    HANDLE h = (is_stderr ? errhandle : outhandle);
     int osize, esize;
 
     assert(len > 0);