Remove Mac/Windows quick_exit() implementation.

This was added with the intention of using it in adb, but then the
change that would have used it
(https://android-review.googlesource.com/c/platform/system/core/+/273824)
was abandoned.

Remove the corresponding (never used) implementation.

Bug: http://b/31468413
Test: treehugger
Change-Id: I42322d079c175b7c6fbd12940e5bc022bd9ebd1c