Remove go.mod

TL;DR: A project with:
- a valid semver tag equal or above v3.0.0
- with a go.mod file
- but not using versioned imports
is not supported by Go in modules mode; GO111MODULE=on on previous
version, and anything but GO111MODULE=off on likely 1.13 or newer when
the default is switched.

The only way to keep it working simultaneously on versions below 1.9.7
and above 1.11 in go module mode is to remove the go.mod file.

It is currently expected that v4.0.0 will be released with a go.mod
file, which will break Go toolchains before 1.9.7. As a note, the
minimum tested version is currently 1.7.6.

Even then, there's some fun. This is because of the source files using
'// +build periphextra'. The tag 'periphextra' only has effect when
building executables under cmd/ and experimental/cmd/. When set, these
executables call hostextra.Init() from periph.io/x/extra/hostextra
instead of host.Init() from periph.io/x/periph/host. The reason is that
periph.io/x/extra contains all drivers that have external dependencies,
while periph.io/x/periph itself aims to not have any external
dependencies. Yet, it's useful to be able to use, let's say, a FT232H
with cmd/gpio-read. Hence the optional build tag.

The problem is that then go mod complains, because it sees the
periph.io/x/extra/hostextra import, and wants to add it as a dependency
in go.sum. Hello circular dependency! I'm not yet sure how I'll handle
this beside hacking manually the go.sum file or removing this option at
all.