Remove conscrypt SDK prebuilts.

They are being moved to prebuilts/module_sdk/conscrypt for consistency
with all other modules.

Also remove the line from mainline/update.py that would have re-added
them next time it is run.

Test: Treehugger
Bug: 188427719
Merged-In: I8a0e9b92f9e74a2d30f0acc1c901a1aa97af61ff
Change-Id: I955cff556c676a6f1c355a6edd5e868daee671d4