Remove unnecessary adbd permissions.

Test: adbd_test (with and without adb root)
  Note: one test fails without root with and without this change
        because of an unrelated shell selinux denial.
Test: adb screencap, pull, and verify
Test: Android Studio screenshot
Bug: 36643190
Change-Id: Ib534240bc9bb3a1f32b8865ca66db988902a0f4a