tools/ocaml: cleanup to reduce compiler warnings

tools/ocaml: cleanup to reduce compiler warnings

This commit cleans up code to reduce compiler warnings:

* remove unused open statements (warning 33)
* remove unused 'rec' declarations (warnign 39)
* remove unused type declarations (warning 34)
* mark unused variables with an underscore (warning 27)
* mark unused value declarations with an underscore (warning 32)

This commit does not include changes to fix compiler warnings 52
(matching against strings in exceptions). These changes have no impact
on functionality.

Signed-off-by: Christian Lindig <christian.lindig@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>