Installation instructions: the optional `cabal install` step confuses users

[andreasabel]

Consider deleting this step from the installation instructions:

agda-stdlib/doc/installation-guide.md

Lines 41 to 46 in 92b3269

	4. [ OPTIONAL ] If using [cabal](https://www.haskell.org/cabal/) then run
	the commands to install via cabal:
	```
	cd agda-stdlib-2.3
	cabal install
	```

I do not think this step has any benefit for the end-users.
Apparently, it confused one of my students: they asked whether this step is necessary if one used cabal to install Agda.