Rename misused "deprecation" to removal (#10000)

# Description
In the past we named the process of completely removing a command and
providing a basic error message pointing to the new alternative
"deprecation".

But this doesn't match the expectation of most users that have seen
deprecation _warnings_ that alert to either impending removal or
discouraged use after a stability promise.

# User-Facing Changes
Command category changed from `deprecated` to `removed`

crates/nu-command/src/removed/let_env.rs

@@ -0,0 +1,43 @@
+use nu_protocol::ast::Call;
+use nu_protocol::engine::{Command, EngineState, Stack};
+use nu_protocol::{Category, PipelineData, ShellError, Signature, SyntaxShape, Type};
+
+#[derive(Clone)]
+pub struct LetEnv;
+
+impl Command for LetEnv {
+    fn name(&self) -> &str {
+        "let-env"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build(self.name())
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
+            .allow_variants_without_examples(true)
+            .optional("var_name", SyntaxShape::String, "variable name")
+            .optional(
+                "initial_value",
+                SyntaxShape::Keyword(b"=".to_vec(), Box::new(SyntaxShape::MathExpression)),
+                "equals sign followed by value",
+            )
+            .category(Category::Removed)
+    }
+
+    fn usage(&self) -> &str {
+        "`let-env FOO = ...` has been removed, use `$env.FOO = ...` instead."
+    }
+
+    fn run(
+        &self,
+        _: &EngineState,
+        _: &mut Stack,
+        call: &Call,
+        _: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        Err(nu_protocol::ShellError::RemovedCommand(
+            self.name().to_string(),
+            "$env.<environment variable> = ...".to_owned(),
+            call.head,
+        ))
+    }
+}