internal/goexperiment: remove the synctest GOEXPERIMENT

synctest package is enabled by default and the synctest
goexperiment does nothing after CL 709355.

Change-Id: Ia96b070d5f3779ae7c38a9044f754e716a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/709555
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: mateusz834

src/internal/goexperiment/exp_synctest_off.go

@@ -100,9 +100,6 @@ type Flags struct {
 	// inlining phase within the Go compiler.
 	NewInliner bool
 
-	// Synctest enables the testing/synctest package.
-	Synctest bool
-
 	// Dwarf5 enables DWARF version 5 debug info generation.
 	Dwarf5 bool