Account for change from unmovable to undeletable only [finishes #71353760]

1. Change callout text + tests which mention unmovability to mention only undeletability
2. Change level instruction text which mentions unmovability to mention only undeletability
3. Remove unmovable block drag tests

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