plugins.c: cleanup plugins_scan_dir()

 add_plugin() no longer returns ENOMEM. Remove code in
 plugins_scan_dir() which printed an error message
 if ENOMEM returned by add_plugin().

 Fix comment re add_plugin() return values.

Change-Id: Id5170c28e2a55884d41ceebc4e07f16389b75a2a
Reviewed-on: https://code.wireshark.org/review/3774
Reviewed-by: Bill Meier <wmeier@newsguy.com>