sync: remove synchashtriemap GOEXPERIMENT

It's been true by default for a full release. It's time to remove the
old map.

Change-Id: I65507f07725e0084aabd389f37d73ade0b7dc35b
Reviewed-on: https://go-review.googlesource.com/c/go/+/690395
Auto-Submit: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: mknyszek

src/internal/buildcfg/exp.go

@@ -83,7 +83,6 @@ func ParseGOEXPERIMENT(goos, goarch, goexp string) (*ExperimentFlags, error) {
 		RegabiArgs:      regabiSupported,
 		AliasTypeParams: true,
 		SwissMap:        true,
-		SyncHashTrieMap: true,
 		Dwarf5:          dwarf5Supported,
 	}
 

src/internal/goexperiment/exp_synchashtriemap_off.go

@@ -115,9 +115,6 @@ type Flags struct {
 	// SwissMap enables the SwissTable-based map implementation.
 	SwissMap bool
 
-	// SyncHashTrieMap enables the HashTrieMap sync.Map implementation.
-	SyncHashTrieMap bool
-
 	// Synctest enables the testing/synctest package.
 	Synctest bool