Remove support for unused env vars from sdk snapshot generation

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