Remove obsolete support for --preload-package in zygotes.

The WebView zygote used to use --preload-package but the last use of
this was removed in 2019, so just delete the code related to this unused
feature (which was only ever a hidden API).

Flag: NONE cleanup of obsolete hidden API
Test: m checkbuild
Change-Id: I1bc83ad6b0175d335d7c4e8c167f266c07e47708