Revert "into cellpath command (#7417)" (#7523)

This reverts commit f0e93c2fa9 (PR #7417).

I'm currently [working on improving cell
paths](https://github.com/nushell/nushell/issues/7498#issuecomment-1356834798),
and I realized that I would need to make several improvements to `into
cellpath` along the lines of Jakub's comment here:
https://github.com/nushell/nushell/pull/7417#issuecomment-1345264955

I don't think `into cellpath` is quite ready for prime-time, and I'd
like to remove it before the upcoming release.

crates/nu-command/tests/commands/into_cellpath.rs

@@ -1,14 +0,0 @@
-use nu_test_support::{nu, pipeline};
-
-#[test]
-fn into_pathcell_string() {
-    let actual = nu!(
-        cwd: ".", pipeline(
-        r#"
-        'nu.is.awesome' | into cellpath
-        "#
-    ));
-    dbg!(&actual.out);
-
-    assert!(actual.out.contains("nu.is.awesome"));
-}

crates/nu-command/tests/commands/mod.rs

@@ -35,7 +35,6 @@ mod headers;
 mod help;
 mod histogram;
 mod insert;
-mod into_cellpath;
 mod into_filesize;
 mod into_int;
 mod last;