Remove local replace directive from go.mod

[elias-orijtech]

Replace directives disable external go run ... support:

% go run github.com/tulir/whatsmeow/mdtest@latest
go: github.com/tulir/whatsmeow/mdtest@latest (in github.com/tulir/whatsmeow/mdtest@v0.0.0-20221126173344-e660988acdbc):
	The go.mod file for the module providing named packages contains one or
	more replace directives. It must not contain directives that would cause
	it to be interpreted differently than if it were the main module.

A better way to develop multiple modules concurrently is with Go 1.18 workspaces[0].

[0] https://go.dev/blog/get-familiar-with-workspaces

[tulir]

Go workspaces don't work very nicely and mdtest should always use the latest version of whatsmeow, so I'll keep it as is