test: remove flaky status for test-npm-install

I haven't seen test-npm-install fail on arm in a long time. There is no
open issue for it being unreliable. Remove flaky designation in
parallel.status file. (If it turns out it is flaky, it's easy enough to
open an issue and restore the flaky designation.)

Ref: nodejs/node#13498

PR-URL: nodejs/node#19216
Refs: nodejs/node#13498
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/parallel.status

@@ -15,7 +15,6 @@ test-postmortem-metadata: PASS,FLAKY
 [$system==macos]
 
 [$arch==arm || $arch==arm64]
-test-npm-install:      PASS,FLAKY
 
 [$system==solaris] # Also applies to SmartOS