simplify install instructions assuming Go 1.16+

Go 1.16 was released nearly a year ago,
and Go 1.15 and earlier are no longer supported by upstream.
Simplify the install steps by no longer mentioning GO111MODULE.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I7f4a8fd7dd247d2b9c96e35ecae2c195dc68fe75
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/527046
Reviewed-by: Paul Jolly <[email protected]>
Unity-Result: CUEcueckoo <[email protected]>

Author: mvdan

README.md

@@ -107,13 +107,7 @@ Using [Homebrew](https://brew.sh), you can install using the CUE Homebrew tap:
 
 <!-- Keep the following in sync with cmd/cue/cmd/testdata/script/install*.txt -->
 
-If you already have Go installed, the short version is:
-
-```
-GO111MODULE=on go get cuelang.org/go/cmd/cue
-```
-
-Or, if you are using Go 1.16:
+If you already have Go 1.16 or later installed, the short version is:
 
 ```
 go install cuelang.org/go/cmd/cue@latest

doc/install.md

@@ -14,16 +14,12 @@ Go 1.14 or higher (see below)
 
 To download and install the `cue` command line tool run
 
-```
-GO111MODULE=on go get cuelang.org/go/cmd/cue
-```
-
-Or, if you are using Go 1.16:
-
 ```
 go install cuelang.org/go/cmd/cue@latest
 ```
 
+If the command fails, make sure your version of Go is 1.16 or later.
+
 And make sure the install directory is in your path.
 
 To also download the API and documentation, run