Remove the redundant/wrong write of 'recovery_as_boot='.

The deleted lines contain a bug that only writes "recovery_as_boot="
into META/misc_info.txt when the value is empty. The issue has no real
impact though, because a) it's no-op for targets not using
BOARD_USES_RECOVERY_AS_BOOT (e.g. non-A/B targets); b) for targets with
BOARD_USES_RECOVERY_AS_BOOT := true, the value gets written correctly as
part of the call to 'generate-userimage-prop-dictionary'.

Test: `m dist` on aosp_bullhead-userdebug. The line is gone from
      META/misc_info.txt.
Test: `m dist` on aosp_marlin-userdebug. 'recovery_as_boot=true' exists
      in META/Misc_info.txt.
Change-Id: I9b4d70d457e141aa308ba2e62e2033602c8012de