[crypto] Add facility to delete unexportable keys

The macOS unexportable keys implementation is stateful. This CL adds a
method deletes macOS keys. The method is a no-op on other
implementations.

The unexportable keys metrics code is also updated to make use of the
new method. The other two existing clients (device trust & dbsc)
currently only use unexportable keys on Windows, and will have to be
updated separately to support deletion to enable the feature on Mac.

Bug: 326451282
Change-Id: Ia7b5252943a3c25e2af859cd862c14e4659dd99e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5352710
Commit-Queue: Nina Satragno <nsatragno@chromium.org>
Reviewed-by: Sébastien Lalancette <seblalancette@chromium.org>
Reviewed-by: Adam Langley <agl@chromium.org>
Auto-Submit: Nina Satragno <nsatragno@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1270413}