idmap2: silence irrelevant warning during 'idmap2 scan'

Even if an idmap file fails the is-up-to-date check during 'idmap2
scan', there is no need to print a warning: this is an expected scenario
and just means that the idmap file needs to be re-created. Remove the
print.

Useful commands for testing:

  1. adb shell rm -f /data/resource-cache/*
  2. adb shell idmap2 scan                                         \
             --recursive                                           \
             --target-package-name android                         \
             --target-apk-path /system/framework/framework-res.apk \
             --output-directory /data/resource-cache               \
             --input-directory /vendor/overlay

Test: manual: put an overlay package with isStatic="true" and       \
              targetPackage="android" in /vendor/overlay/<dir>, run \
              the commands above, verify that nothing is printed    \
              on stderr
Change-Id: I9a458a805936d0c1f6d64b0c3983365e0f0c4696