Remove index arguments and state point backups

[bdice]

Description

Additional cleanup related to #588. This PR removes index arguments from a number of methods of Project and the CLI.

I also removed the public methods read_statepoints, write_statepoints, and dump_statepoints from the public API (see #579).

Finally, I removed the deprecated function get_statepoint from the public API because it used FN_STATEPOINTS which was removed as a part of the previously mentioned read/write/dump_statepoints changes. I would have split this into two pull requests but there would have been a couple annoying merge conflicts. Since this is a pure removal, I thought it would be fine to combine the two sets of changes to avoid having to deal with the conflicts.

Deprecations for the index argument will need to be added to signac 1.x.

Motivation and Context

signac 2.0 API cleanup. There is no longer a public method for generating an index, so it no longer makes sense to allow users to supply one as an argument.

Types of Changes

• Documentation update
• Bug fix
• New feature
• Breaking change¹

¹The change breaks (or has the potential to break) existing functionality.

Checklist:

• I am familiar with the Contributing Guidelines.
• I agree with the terms of the Contributor Agreement.
• My name is on the list of contributors.
• My code follows the code style guideline of this project.
• The changes introduced by this pull request are covered by existing or newly introduced tests.

If necessary:

• I have updated the API documentation as part of the package doc-strings.
• I have created a separate pull request to update the framework documentation on signac-docs and linked it here.
• I have updated the changelog and added all related issue and pull request numbers for future reference (if applicable). See example below.

[codecov]

Codecov Report

Merging #599 (62034d8) into next (5f3d162) will increase coverage by 0.19%.
The diff coverage is 100.00%.

❗ Current head 62034d8 differs from pull request most recent head 0a490d0. Consider uploading reports for the commit 0a490d0 to get more accurate results

@@            Coverage Diff             @@
##             next     #599      +/-   ##
==========================================
+ Coverage   85.32%   85.52%   +0.19%     
==========================================
  Files          52       52              
  Lines        5207     5120      -87     
  Branches      957      928      -29     
==========================================
- Hits         4443     4379      -64     
+ Misses        551      540      -11     
+ Partials      213      201      -12     

Impacted Files	Coverage Δ
signac/contrib/filterparse.py	88.04% <ø> (+3.50%)	⬆️
signac/__main__.py	75.75% <100.00%> (+0.57%)	⬆️
signac/contrib/import_export.py	77.83% <100.00%> (-0.06%)	⬇️
signac/contrib/linked_view.py	89.60% <100.00%> (+1.45%)	⬆️
signac/contrib/project.py	86.34% <100.00%> (+0.06%)	⬆️

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