build: remove pnpm engine value due to lack of yarn support

Yarn displays a warning due to an unknown engine value of `pnpm`.  Until this is corrected upstream, the value can be temporarily removed to eliminate the multiple, but otherwise harmless, warnings shown to users upon package install.

Author: clydin

lib/packages.ts

@@ -84,7 +84,6 @@ function loadPackageJson(p: string) {
         pkg['engines'] = {
           'node': '>= 10.13.0',
           'npm': '>= 6.11.0',
-          'pnpm': '>= 3.2.0',
           'yarn': '>= 1.13.0',
         };
         break;