[DRE-maint] Bug#1019924: Remove build dependency on ruby-blade

[Pirate Praveen]

Package: src:rails
Version: 2:6.1.7+dfsg-1
Severity: important
Control: forwarded -1 https://github.com/rails/rails/pull/45546

ruby-blade depends on now deprecated ruby-coffee-script and rails is 
only reverse build dependency of ruby-blade. Upstream has already moved 
to rollup, filing this to track progress of ruby-blade removal.