remove old mv command in favor of umv (renamed to mv) (#12022)

# Description

This PR removes our old nushell `mv` command in favor of the
uutils/coreutils `uu_mv` crate's `mv` command which we integrated in
0.90.1.

# User-Facing Changes

# Tests + Formatting

# After Submitting

crates/nu-command/src/default_context.rs

@@ -207,7 +207,6 @@ pub fn add_shell_command_context(mut engine_state: EngineState) -> EngineState {
             Ls,
             UMkdir,
             Mktemp,
-            Mv,
             UMv,
             UCp,
             Open,