The --snapshot argument was only necessary in Dart 1. The --depfile
argument was only used in Dart 2 mode to pass to the kernel compiler,
but was inconsistent with the 'build aot' command, where the depfile was
always set to build/kernel_compile.d.

This patch updates 'build bundle' to emit the depfile to a location
consistent with the 'build aot' command; since it's not intended to be
user-configurable and flutter.gradle hardcodes the location to
build/kernel_compile.d either way, this patch also eliminates the
ability to configure the filename altogether.