usb: musb: remove 'ignore_disconnect' flag

This was related to an old bug on early versions
of TUSB6010 which we don't support anymore.

It's known to cause issues on several other
situations with more recent devices so we
better remove this flag now and come up
with a better workaround should one be deemed
necessary.

Signed-off-by: Felipe Balbi <balbi@ti.com>