remove Sourcegraph70 feature flag and stale extensions/gateway help entries

Sourcegraph70 was only used by the teams version gate. The `extensions` and
`gateway` commands were removed in #1173 and earlier but were still listed in
`src help`.

Part of https://linear.app/sourcegraph/issue/FE-502

Amp-Thread-ID: https://ampcode.com/threads/T-01a08410-86ca-72be-9928-2810e837fae1
Co-authored-by: Amp <amp@ampcode.com>

Author: marcleblanc2

cmd/src/main.go

@@ -58,9 +58,7 @@ The commands are:
 	batch           manages batch changes
 	code-intel      manages code intelligence data
 	config          manages global, org, and user settings
-	extensions,ext  manages extensions (experimental)
 	extsvc          manages external services
-	gateway         interacts with Cody Gateway
 	login           authenticate to a Sourcegraph instance with your user credentials
 	orgs,org        manages organizations
 	repos,repo      manages repositories

internal/features/features.go

@@ -13,7 +13,6 @@ import (
 type FeatureFlags struct {
 	Sourcegraph40 bool
 	BinaryDiffs   bool
-	Sourcegraph70 bool
 }
 
 func (ff *FeatureFlags) SetFromVersion(version string, skipErrors bool) error {
@@ -33,7 +32,6 @@ func (ff *FeatureFlags) SetFromVersion(version string, skipErrors bool) error {
 		// {&ff.FlagName, ">= 3.23.0-0", "2020-11-24"},
 		{&ff.Sourcegraph40, ">= 4.0.0-0", "2022-08-24"},
 		{&ff.BinaryDiffs, ">= 4.3.0-0", "2022-11-29"},
-		{&ff.Sourcegraph70, ">= 7.0.0-0", "2026-02-25"},
 	} {
 		value, err := api.CheckSourcegraphVersion(version, feature.constraint, feature.minDate)
 		if err != nil {