Outdated docs: available_modules.go

[kpcyrd]

available_modules.go is mentioned in some places in the docs, but the file doesn't seem to exist anymore:

$ rg available_modules
doc/modules.rst
127:``conf/available_modules.go``. You should use this file to add or remove modules.

modules/fswatch/doc.rst
37:In ``available_modules.go``, make sure the module is enabled when the agent is

modules/yara/doc.rst
45:enabled in `conf/available_modules.go` (or whatever you have the Makefile variable
$ 

I think this was replaced with modulepack.