remove: pdm-style call command

[metaist]

The idea behind call commands comes from pdm. It's a nice short-hand where you write module:func and it turns into a python call.

Some issues:

1. Not transparent: module:func becomes python -c 'import sys, module as _1; sys.exit(_1.func())'. The difference between what you write and what you get is pretty big.
2. Requires Python: Eventually I want to remove python as a dependency. This keeps it baked in.
3. No argument interpolation: pdm doesn't support injecting arguments into this kind of command.
4. No error suppression: This really is a very different kind of thing from the rest of the types of commands.

Removing this will make us a little less compatible with pdm, but it's probably worth keeping it simple.