Fix crash on text hint being triggered after disabling code browser plugin

From KTextEditor::TextHintInterface::registerTextHintProvider() docs:
"When the provider is about to be destroyed, make sure to call
unregisterTextHintProvider() to avoid a dangling pointer."
(new requirement with KF5)

The old code never unregistered in case the ContextBrowserPlugin was
deleted (like it happens when disabling the plugin in the settings in
a running KDevelop instance).

CCBUG: 411371