imp: Remove unused `npx` reference (#28544)

Summary:
Recently we removed `npx` usage from `react-native-cli` flow. After checking usages in this repo I found unused reference.

## Changelog

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[Internal] [Removed] - Remove unused `npx` reference
Pull Request resolved: https://github.com/facebook/react-native/pull/28544

Test Plan: Tests pass

Reviewed By: cpojer

Differential Revision: D20873090

Pulled By: hramos

fbshipit-source-id: 12e05e9635a83f19439024766817e4599320af98

react.gradle

@@ -99,7 +99,6 @@ afterEvaluate {
         // Additional node and packager commandline arguments
         def nodeExecutableAndArgs = config.nodeExecutableAndArgs ?: ["node"]
         def extraPackagerArgs = config.extraPackagerArgs ?: []
-        def npx = Os.isFamily(Os.FAMILY_WINDOWS) ? "npx.cmd" : "npx"
 
         def execCommand = []