Remove OS specific install instructions that all repeat the same thing.
Since `go get ...` is being deprecated (to install executables)
recommend an install procedure that works on all supported versions of
go (`git clone && go install`) but also mention the new `go install`
mode in 1.16.
1. Adds a note to the documentation describing how to use 'go get' in
modules mode
2. Removes the '-u' option which, when 'go get' is used incorrectly
will make 'go' try to compile Delve with unsupported dependencies
3. Removed obsolete note about Go 1.5 vendor experiment.
Fixes#1988
