Reverted disableMouseCursor() case to not touch oldMouseLockValid since it doesn't have a clear useful benefit.

This way, the patch is simpler and only affects the use case where it
has a valid benefit: enabling the cursor for long-term while it's
temporarily made visible by GLFW.

If in the future it makes sense for calls to
glfwDisable(GLFW_MOUSE_CURSOR) to reset oldMouseLockValid, then this
can be revisited and done properly (with tests).