Prepare running twice

[gilest]

Since #1031 running local commands like pnpm install causes the addon build to run twice.

This is because we added a workspace-level prepare script while leaving the package-level prepare script in place.

This PR removes the workspace-level prepare script to restore a single build being run.

Example:

➜  ember-file-upload git:(master) p i
ember-file-upload prepare$ pnpm build
[7 lines collapsed]
│ [js] > [email protected] build:js
│ [js] > rollup --config
│ [js]
│ [js]
│ [js]  → dist...
│ [js] (!) Generated an empty chunk
│ [js] "template-registry"
│ [js] created dist in 715ms
│ [js] npm run build:js exited with code 0
│ [types] npm run build:types exited with code 0
└─ Done in 4s
. prepare$ pnpm -F ember-file-upload prepare
[9 lines collapsed]
│ [js] > [email protected] build:js
│ [js] > rollup --config
│ [js]
│ [js]
│ [js]  → dist...
│ [js] (!) Generated an empty chunk
│ [js] "template-registry"
│ [js] created dist in 716ms
│ [js] npm run build:js exited with code 0
│ [types] npm run build:types exited with code 0
└─ Done in 4s
Done in 7.2s

With this change it will run only once again:

➜  ember-file-upload git:(master) ✗ p i
. prepare$ pnpm -F ember-file-upload build
[7 lines collapsed]
│ [types] > [email protected] build:types
│ [types] > glint --declaration
│ [types]
│ [js]
│ [js]  → dist...
│ [js] (!) Generated an empty chunk
│ [js] "template-registry"
│ [js] created dist in 596ms
│ [js] npm run build:js exited with code 0
│ [types] npm run build:types exited with code 0
└─ Done in 2.7s
Done in 3.6s