Makefile: remove long-obsolete -tags=xversion

That build tag hasn't been used since tailscale/tailscale's 5088af68cf
(June 2nd, 2021, for 1.10.0)

Makefile

@@ -59,7 +59,7 @@ $(DEBUG_APK): toolchain
 release_aar: toolchain
 release_aar:
 	mkdir -p android/libs
-	go run gioui.org/cmd/gogio -ldflags "-X tailscale.com/version.Long=$(VERSIONNAME) -X tailscale.com/version.Short=$(VERSIONNAME_SHORT) -X tailscale.com/version.GitCommit=$(TAILSCALE_COMMIT) -X tailscale.com/version.ExtraGitCommit=$(OUR_VERSION)" -tags xversion -buildmode archive -target android -appid $(APPID) -o $(AAR) github.com/tailscale/tailscale-android/cmd/tailscale
+	go run gioui.org/cmd/gogio -ldflags "-X tailscale.com/version.Long=$(VERSIONNAME) -X tailscale.com/version.Short=$(VERSIONNAME_SHORT) -X tailscale.com/version.GitCommit=$(TAILSCALE_COMMIT) -X tailscale.com/version.ExtraGitCommit=$(OUR_VERSION)" -buildmode archive -target android -appid $(APPID) -o $(AAR) github.com/tailscale/tailscale-android/cmd/tailscale
 
 $(RELEASE_AAB): release_aar
 	(cd android && ./gradlew bundlePlayRelease)