fix: remove postinstall script

This was needed a few years ago to let people know that without active
support I'd (remy) wouldn't feel like there's any actual support to help
me maintain this project.

Since then (it's been a good few years), there's been a few things come
up. Firstly, the `||` breaks (some) powershell users. Not all, I could
never replicate the problem myself, but others definitely experience it.

Then there was the `npm fund` attempt to support projects. In honesty,
this does zero to actually support nodemon. The reality is that people
want their tools (and software) for free, so asking users to run an
extra step after installing (the `npm fund` bit) and _then_ pick a
project they want to support *and then* do the payment thing was
just way way too much. I know I don't use it, and I'm positive others
don't too.

The majority of early supporters of nodemon have been individuals, and
I know it was because of the postinstall script I added back in 2018:
https://remysharp.com/2018/01/10/open-source-with-a-cap-in-hand

Lastly, the vast, vast majority of financial supporters to nodemon in
the last 12 months haven't been individuals. Nor have they been
companies (with the exceptions being opencollective themselves and
Frontend Masters - both, thank you, it means a lot). The recent
supporters have come from an…"odd" spot of the internet. Perhaps to
help their SEO ranking, I'm not sure, but I am going to take their
money as a way of them saying "let's support the many developers who
use nodemon by giving it's maintainer financial support".

So, long story short: I've removed the postinstall, and hopefully
this reduces the noise to you, the amazing day-job developer, and
hopefully that odd corner of the web continues their donations, as
I'm positive they're not using the `npm fund`!

package.json

@@ -32,16 +32,14 @@
     "commitmsg": "commitlint -e",
     "coverage": "istanbul cover _mocha -- --timeout 30000 --ui bdd --reporter list test/**/*.test.js",
     "lint": "eslint lib/**/*.js",
-    ":spec": "node_modules/.bin/mocha --timeout 30000 --ui bdd test/**/*.test.js",
     "test": "npm run lint && npm run spec",
     "spec": "for FILE in test/**/*.test.js; do echo $FILE; TEST=1 mocha --exit --timeout 30000 $FILE; if [ $? -ne 0 ]; then exit 1; fi; sleep 1; done",
     "postspec": "npm run clean",
     "clean": "rm -rf test/fixtures/test*.js test/fixtures/test*.md",
     "web": "node web",
     "semantic-release": "semantic-release",
     "prepush": "npm run lint",
-    "killall": "ps auxww | grep node | grep -v grep | awk '{ print $2 }' | xargs kill -9",
-    "postinstall": "node bin/postinstall || exit 0"
+    "killall": "ps auxww | grep node | grep -v grep | awk '{ print $2 }' | xargs kill -9"
   },
   "devDependencies": {
     "@commitlint/cli": "^11.0.0",