cmake: forget about plugins/asn1: it's not used any more.

(Don't load plugins/asn1/plugin.rc.in)

Change-Id: I621552a7e915396f61c5bb450a99d4e575ac30df
Reviewed-on: https://code.wireshark.org/review/3324
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>