Remove explicit failure in WithDisableGlobalRegistry

[mfridman]

Fix #782

This PR updates the behavior of the provider WithDisableGlobalRegistry option, specifically removing this check:

if len(global) > 0 {
	return nil, errors.New("global registry disabled, but provider has registered go migrations")
}

Allowing programs to have multiple providers who may be running in a (legacy) environment with globaly-registered migrations. By removing this check, the caller can now explicitly disable global migrations and isolate go migrations to the current provider with WithGoMigrations.

TL;DR - the combination of WithGoMigrations and WithDisableGlobalRegistry allows you to have a fully isolated provider.

• Decide whether this should log a warning, or simple ignore and do nothing