remove def-env and export def-env (#10999)

follow-up to
- https://github.com/nushell/nushell/pull/10715

> **Important**
> wait for between 0.87 and 0.88 to land this

# Description
it's time for removal again 😋 
this PR removes `def-env` and `export def-env` in favor of `def --env`

# User-Facing Changes
`def-env` and `export def-env` will not be found anymore.

# Tests + Formatting

# After Submitting

crates/nu-parser/src/parser.rs

@@ -5085,7 +5085,7 @@ pub fn parse_builtin_commands(
     let name = working_set.get_span_contents(lite_command.parts[0]);
 
     match name {
-        b"def" | b"def-env" => parse_def(working_set, lite_command, None).0,
+        b"def" => parse_def(working_set, lite_command, None).0,
         b"extern" => parse_extern(working_set, lite_command, None),
         b"let" => parse_let(working_set, &lite_command.parts),
         b"const" => parse_const(working_set, &lite_command.parts),