MB-9307: Size of couch_file:read_header_bin/2 is not used

Returning the size of the header is not needed anywhere.

Change-Id: I9f7f97d0a4e00fcb35635332da3be75dd83b3a01
Reviewed-on: http://review.couchbase.org/30859
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>