Remove fd caching since we don't have unload (deconstructor) support
basiclly this is reverting
164c99a4b414b614e5185a96ef8287331e9134eb
the problem is when an application is using PAM loaded and unloaded
and over again, the file descriptior never get closed on unload of the
pam module. If main app already uses Heimdal, Heimdal doesn't get
unloaded, but in some scenarios this happen more often.
Since we now use fortuna for our internal random generator, this is
not that bad.
Bug found by Victor Guerra.