logd: Stop calling prctl(PR_SET_KEEPCAPS, 1)

This change removes the call to prctl(PR_SET_KEEPCAPS, 1), because now
that logd is being run with ambient capabilities, init will set the
KEEP_CAPS / KEEP_CAPS_LOCKED securebits.

Bug: 110127913
Test: liblog-unit-tests, logd-unit-tests, logcat-unit-tests
      (on an aosp_sailfish-userdebug)

Change-Id: I72b6a49c5cd1552085f9ad44cb19a7333a13eeea