Remove support for blobs snapshots from Dart VM

[mkustermann]

Now that we can write/read ELF we should remove the blobs format by

• [x] Mark the blob related functionality as deprecated, in progress https://dart-review.googlesource.com/c/sdk/+/121704)
• [x] Remove internal uses
• [x] Notify all known embedders of this change
• [ ] Fix embedders we care about ourselves Cast: complete Flutter: complete Fuchsia: in progress
• [ ] Stop testing blobs and remove support completely
• [ ] Make AppJIT snapshots use ELF instead of blobs

[sjindel-google]

For context, we're deprecating blobs in favor of Assembly and ELF snapshots.

The blobs form currently has two significant limitations:

1. We cannot add new sections without updating all the Dart and Flutter embedders.
2. We cannot control or predict the relative positions of sections, inhibiting PC-relative loads and calls.

With Assembly and ELF snapshots we can add new sections without updating the embedders and discover the address of the new sections at runtime by offset from any of the existing sections.