polkitAgent: Reset the session request timeout when removing it

polkitAgent: Reset the session request timeout when removing it

When handing the resetDialog request we're leaving a source ID alive,
leading this error:

(gnome-shell:22464): GLib-CRITICAL **: 17:46:11.065: Source ID 12934 was not
found when attempting to remove it:

== Stack trace for context 0x55c9246916c0 ==
#0 55c9249151b8 i   js/ui/components/polkitAgent.js:391 (11f71fd544c0 @ 100)
#1 7ffc55140aa0 b   self-hosted:1009 (3062ba49af88 @ 423)
#2 55c924915120 i   js/ui/modalDialog.js:167 (1c9e50ae9880 @ 62)
#3 55c924915098 i   js/ui/modalDialog.js:186 (1c9e50ae9970 @ 12)
#4 55c924915008 i   js/ui/environment.js:75 (1c9e50a8d5b0 @ 98)
#5 55c924914f78 i   js/ui/environment.js:149 (1c9e50a8d9e8 @ 14)

So, reset the source handle to avoid trying to remove it.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1203