Potential fix for code scanning alert no. 3759: Unused variable, import, function or class

Co-authored-by: Copilot Autofix powered by AI <62310815+github-advanced-security[bot]@users.noreply.github.com>

src/extras/PMREMGenerator.js

@@ -675,7 +675,6 @@ function _createPlanes( lodMax ) {
 				// logic matching _getCommonVertexShader
 				const x = u * 2 - 1;
 				const y = v * 2 - 1;
-				const z = 1;
 
 				if ( face === 0 ) {