Change unmovable blocks to undeletable only [finishes #71353760]

Large find + replace and regex-based cleanup job. Remove all unmovable blocks from our XML-defined levels, their tests + required blocks.

Previously unmovable blocks should already be set to undeletable.

See also:

https://github.com/code-dot-org/dashboard/pull/698
https://github.com/code-dot-org/blockly/pull/601
https://github.com/code-dot-org/blockly-core/pull/85