Clean up our depedencies install code

[infomiho]

After we migrated to using npm worksapces (#3130) a lot of our custom npm install and dependency sharing logic became unnecessary.

For example, check the comment above this function: https://github.com/wasp-lang/wasp/blob/main/waspc/src/Wasp/Generator/NpmInstall.hs#L30

It says it runs npm install three times, but in reality it only runs it once (since with npm workspaces we only need one npm install and npm will figure out the rest).

Another example is here: https://github.com/wasp-lang/wasp/blob/main/waspc/src/Wasp/Generator/NpmDependencies.hs#L85

We check for conflicts before running the npm install command even though we already checked for conflicts when generating the package.json files for server and web-app.

We should dig into the this part of the codebase and figure out if there is more code to refactor. Related effort in this PR: #3477