macdeploy: remove unused plistlib import

Unused since #20422, see: https://github.com/bitcoin/bitcoin/pull/20422#discussion_r534207899

contrib/macdeploy/macdeployqtplus

@@ -16,7 +16,6 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #
 
-import plistlib
 import sys, re, os, shutil, stat, os.path
 from argparse import ArgumentParser
 from ds_store import DSStore