ANDROID: Revert "ANDROID: GKI: gki_defconfig: CONFIG_DM_DEFAULT_KEY=m"

This reverts commit 7fc2fedd.

There's actually no need for vendors to override dm-default-key.

Bug: 154836329
Bug: 156673772
Change-Id: Id7fd2b970f0526949934102b9aba48d3a12f579b
Signed-off-by: Eric Biggers <ebiggers@google.com>