bootstat: remove unused FDE boot events

Android 13 dropped support for Full Disk Encryption (FDE) entirely, as
it has been replaced by File Based Encryption (FBE).  Therefore, the
FDE-specific boot events are never emitted.  Remove them from the code:

   * boot_decryption_complete
   * boot_complete_post_decrypt
   * factory_reset_boot_complete_post_decrypt
   * ota_boot_complete_post_decrypt
   * post_decrypt_time_elapsed

Bug: 208476087
Change-Id: Id7a3bbd4f273693bd37b0cd27542dc6080d1c4f4