gdb/continuations: do minor cleanup

gdb/continuations: do minor cleanup

Inferior continuations are no longer used by the until and finish
command.  It is used only by the attach command and the remote target
upon detecting new inferiors.  Update the comment accordingly.

Also update another comment about non-existent thread continuations and
remove an unused #include.

gdb/ChangeLog:
2021-04-22  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* continuations.h: Update the general comment.
	* inferior.h (class inferior) <continuations>: Update the comment.
	* interps.c: Do not include "continuations.h".