Remove installDeps

[milesfrain]

This step was leftover from when we used pnpm.

purescript-cookbook/makefile

Lines 93 to 98 in 1d3ede9

	# Downloads all dependencies and sets up cookbook, so that end-user doesn't
	# accidentally use `npm`.
	.PHONY: installDeps
	installDeps:
	> npm i
	> rm package-lock.json

Proposing:

• Remove installDeps from makefile
• Add package-lock.json to .gitignore
• Update readmes to mention npm i for installing dependencies locally.