Remove unused SyntaxShape::Variable (#10511)

# Description
We don't use this shape during parsing and never reference it in command
signatures. Thus it should be removed.

# User-Facing Changes
None functional.
Plugin authors that used it would never be provided with data that
specifically matched `SyntaxShape::Variable`
Builds using it will now fail.

# Tests + Formatting
NA

crates/nu-parser/src/parser.rs

@@ -2743,7 +2743,6 @@ pub fn parse_shape_name(
         b"signature" => SyntaxShape::Signature,
         b"string" => SyntaxShape::String,
         _ if bytes.starts_with(b"table") => parse_collection_shape(working_set, bytes, span),
-        b"variable" => SyntaxShape::Variable,
         b"var-with-opt-type" => SyntaxShape::VarWithOptType,
         _ => {
             if bytes.contains(&b'@') {