update lintr config for lintr >= 2

[russHyde]

Prework

• [+ ] I understand and agree to the code of conduct and the contributing guidelines.
• [+] I have already submitted a discussion topic or issue to discuss my idea with the maintainer.
• [+] This pull request is not a draft.

Related GitHub issues and pull requests

• Ref: # https://github.com/jimhester/lintr/blob/master/NEWS.md#lintr-200 - deprecation of snake_case_linter and camel_case_linter

• .lintr config is for lintr < v2 #295

Summary

Please explain the purpose and scope of your contribution.

The package was set to use CRAN {lintr} (ie, >=v2)

snake_case_linter and camel_case_linter were originally NULL-ed out (deactivated) in the
{targets} .lintr config.

But, these linters were removed from lintr's default-linter set prior to
lintr v2 (so NULLing them in the config doesn't actually do anything).
In fact, the functions are being deprecated.

Hence I removed any mention of these linters from the targets .lintr.

Note that snake_case_linter / camel_case_linter are now implemented as
part of object_name_linter(), but that the latter was already deactivated in
the {targets} .lintr config.

[wlandau]

Thanks, @russHyde.