ci: drop support for Go 1.13 and 1.14

They aren't supported anymore in CI, and because untested code is broken
code, let's remove support for these Go versions altogether.

tests/wasm/event_test.go

@@ -1,5 +1,3 @@
-// +build go1.14
-
 package wasm
 
 import (

tests/wasm/fmt_test.go

@@ -1,14 +1,5 @@
-// +build go1.14
-
 package wasm
 
-// NOTE: this should work in go1.13 but panics with:
-// panic: syscall/js: call of Value.Get on string
-// which is coming from here: https://github.com/golang/go/blob/release-branch.go1.13/src/syscall/js/js.go#L252
-// But I'm not sure how import "fmt" results in this.
-// To reproduce, install Go 1.13.x and change the build tag above
-// to go1.13 and run this test.
-
 import (
 	"testing"
 	"time"

tests/wasm/fmtprint_test.go

@@ -1,5 +1,3 @@
-// +build go1.14
-
 package wasm
 
 import (

tests/wasm/log_test.go

@@ -1,5 +1,3 @@
-// +build go1.14
-
 package wasm
 
 import (