roachprod: remove COCKROACH_UI_RELEASE_NOTES_SIGNUP_DISMISSED

Remove COCKROACH_UI_RELEASE_NOTES_SIGNUP_DISMISSED which
had previously been used by roachprod circa 2020, until it
got reverted in [1]. It has no effect after the PR which reverted
the related change.

Release note: None
Epic: None

[1] #57003

pkg/roachprod/config/config.go

@@ -86,8 +86,6 @@ func DefaultEnvVars() []string {
 		// when moving snapshots around, though.
 		// (For other perf. related knobs, see https://github.com/cockroachdb/cockroach/issues/17165)
 		"COCKROACH_ENABLE_RPC_COMPRESSION=false",
-		// Get rid of an annoying popup in the UI.
-		"COCKROACH_UI_RELEASE_NOTES_SIGNUP_DISMISSED=true",
 		// Allow upgrading a stable release data-dir to a dev version.
 		// N.B. many roachtests which perform upgrade scenarios require this env. var after changes in [1]; otherwise,
 		// the tests will fail even on release branches when attempting to upgrade previous (stable) release to an alpha.