distbuild: Remove the build-steps message

This message was hundreds of kilobytes in size, as it contained a
recursive list of dependencies for each artifact in the build graph. It
was used in the initiator only to print this message:

    Build steps in total: 592

This message is now gone. The 'Need to build %d artifacts'
build-progress message now indicates the total build steps instead:

    Need to build 300 artifacts, of 592 total

This is a compatible change to the distbuild protocol: old initiators
will continue to work as normal with new controllers that don't send
the build-steps message.

1 files changed, 0 insertions, 4 deletions

diff --git a/distbuild/protocol.py b/distbuild/protocol.py
index 8af84960..dee45d17 100644
--- a/distbuild/protocol.py
+++ b/distbuild/protocol.py
@@ -30,10 +30,6 @@ _required_fields = {
         'id',
         'message',
     ],
-    'build-steps': [
-        'id',
-        'steps',
-    ],
     'step-started': [
         'id',
         'step_name',