Remove asv dev command

[datapythonista]

The asv dev command is confusing, and made few projects pin to asv 0.4 after 0.5 changed it's behavior.

I'm going to remove it. In asv 0.6, I'll keep the command, but making it fail with a descriptive error message that asv run should be used instead, and then it can be removed in 0.7