fix(#960): drop unused postinstall and patch-package

[Thayorns]

The postinstall script and the patch-package devDependency were left over from when the project applied patches/grunt-mocha-cli+7.0.0.patch. That patch set spawnOptions.opts.shell = true (and appended a .cmd suffix) on Windows because grunt-mocha-cli spawned mocha as a separate child process, which needed a shell on Windows.

The project now runs tests through grunt-mocha-test, which loads and runs mocha in-process (new Mocha(...) then mocha.run() in tasks/lib/MochaWrapper.js). There is no child_process, no spawn, and no shell involved, so the Windows-shell fix has nothing to apply to and is not needed.

Since no patch is required, this removes the now-empty scripts/postinstall.js, its postinstall entry in package.json, and the unused patch-package devDependency (with the corresponding package-lock.json cleanup).

Closes #960.

[Thayorns]

@yegor256 take a look please

[Thayorns]

@yegor256 review please