Code in AudioIdent::setIdent() expects that SGSampleGroup::remove() does
remove the corresponding item from _samples, however SG commit 47aae88ab
broke this assumption, which caused the bug reported at [1].
Restore the immediate removal from _samples by default. For the special
case where we do need delayed removal (i.e., while
SGSampleGroup::update() is iterating over _samples), the new optional
parameter is passed the value true. For a longer explanation, see [2].
This fixes a crash when swapping the selected and standby frequencies
for the NAV1 or NAV2 instrument (more precise description at [1]).
[1] https://forum.flightgear.org/viewtopic.php?f=25&t=40890
[2] https://sourceforge.net/p/flightgear/mailman/message/37721699/