update lintr config for lintr >= 2

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

`snake_case_linter` and `camel_case_linter` were NULL-ed out in the
.lintr config.

But, these linters were removed from lintr's default-linter set prior to
lintr v2 (so NULLing them has no effect).

.lintr

@@ -1,7 +1,5 @@
 linters: with_defaults(
-  camel_case_linter = NULL,
   cyclocomp_linter = NULL,
   object_length_linter = NULL,
   object_name_linter = NULL,
-  object_usage_linter = NULL,
-  snake_case_linter = NULL)
+  object_usage_linter = NULL)