Add ability to remove conda-store packages

[peytondmurray]

Summary

This PR adds the ability to remove packages from a conda-store environment.

Changes

• Added a function which removes packages from the currently selected environment. Conda-store handles environment changes differently from regular conda; instead of modifying an environment, a new build (i.e. new environment) is created which includes the requested modifications. To achieve this, the new functions introduced in this PR do the following:

1. Query the conda-store server to gather all the installed packages of the environment
2. Remove the requested packages from the list of installed packages to create a new package list
3. Submit a new build specification with the new package list

Note that changes to the environment take a little time to be digested by the conda-store server, so deleted packages may persist for a little while until the build completes. Once the build finishes, the removed packages will be displayed correctly when the package list is refreshed.

[github-actions[bot]]

:point_left: Launch a binder notebook on the branch Quansight/gator/remove-conda-store-packages