cue/build: get rid of ParseOptions

Pretty much only used for adding comments,
which is always done. Done by default now.

Not parsing comments by default also led to
a bunch of gotchas.

Also get rid of PurgeCache

Change-Id: I0f2be529814a356d2403e5c3af3c7ee89b541892
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2366
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/encoding/openapi/openapi_test.go b/encoding/openapi/openapi_test.go
index 2376a40..33a15e6 100644
--- a/encoding/openapi/openapi_test.go
+++ b/encoding/openapi/openapi_test.go
@@ -23,18 +23,12 @@
 	"testing"
 
 	"cuelang.org/go/cue"
-	"cuelang.org/go/cue/build"
 	"cuelang.org/go/cue/load"
-	"cuelang.org/go/cue/parser"
 	"github.com/kylelemons/godebug/diff"
 )
 
 var update *bool = flag.Bool("update", false, "update the test output")
 
-var config = &load.Config{
-	Context: build.NewContext(build.ParseOptions(parser.ParseComments)),
-}
-
 func TestParseDefinitions(t *testing.T) {
 	defaultConfig := &Config{}
 	resolveRefs := &Config{ExpandReferences: true}
@@ -59,7 +53,7 @@
 		t.Run(tc.out, func(t *testing.T) {
 			filename := filepath.Join("testdata", filepath.FromSlash(tc.in))
 
-			inst := cue.Build(load.Instances([]string{filename}, config))[0]
+			inst := cue.Build(load.Instances([]string{filename}, nil))[0]
 
 			b, err := Gen(inst, tc.config)
 			var out = &bytes.Buffer{}