The following environment variables are no longer used when generating
sdk snapshots:
* SOONG_SDK_SNAPSHOT_PREFER - we can control the modules that are
preferred at build time now.
* SOONG_SDK_SNAPSHOT_USE_SOURCE_CONFIG_VAR - this is done as a
post-processing step by the mainline_module_sdks.sh script to avoid
having to regenerate the ninja files for every module.
* SOONG_SDK_SNAPSHOT_VERSION - support for this was removed a long time
ago but was still referenced from dumpvars.go.
Bug: 259095197
Test: m nothing
# Searched for uses of these variables but could not find any
# outside the build.
Change-Id: Id2b32b29ff6f09faffea43292f549ba13acd956d
82d75ade2a