Do not raise if record is already removed (e.g. with --deps-only)

[jaimergp]

Description

Closes #613

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[jaimergp]

pre-commit.ci autofix

[jaimergp]

I had to add a new block of code that essentially contradicts this comment:

# Using a special instance of PrefixGraph to remove youngest child nodes that match
# the original requested specs.  It's important to remove only the *youngest* child
# nodes, because a typical use might be `conda install --only-deps python=2 flask`,
# and in that case we'd want to keep python.
#
# What are we supposed to do if flask was already in the environment?
# We can't be removing stuff here that's already in the environment.
#
# What should be recorded for the user-requested specs in this case? Probably all
# direct dependencies of flask.

I think we should actually NOT keep python, but I special cased it. My intuition as a user would be like conda install --only-deps A B C shouldn't leave any of A, B, C in the final solution, even if C depends on B. The user made the effort of writing it down, so it must be important. If they really need to condition the solver with more constraints, then they should do it in more steps or, you know, conda remove --force their way there.