Remove unused import for isUndefined

Signed-off-by: Gururaj Gurram <143020143+gururaj1512@users.noreply.github.com>

Author: gururaj1512

lib/node_modules/@stdlib/plot/vega/compare/ctor/lib/field/set.js

@@ -23,7 +23,6 @@
 // MODULES //
 
 var logger = require( 'debug' );
-var isUndefined = require( '@stdlib/assert/is-undefined' );
 var isString = require( '@stdlib/assert/is-string' ).isPrimitive;
 var isStringArray = require( '@stdlib/assert/is-string-array' ).primitives;
 var isSignalReference = require( '@stdlib/plot/vega/base/assert/is-signal-reference' );