adb: stop checking for CryptKeeper mode

Since support for Full Disk Encryption was removed in favor of
File Based Encryption, "CryptKeeper mode" no longer exists and the
"vold.decrypt" system property is never set.  Therefore, remove the
unused code from AdbDebuggingManager that deferred the adb confirmation
in CryptKeeper mode.

Bug: 208476087
Change-Id: I1f289e1257115b51af4eb12ea45734831cc2d0cf