Revert "v4l2codecs: decoder: Unmark previously pending request"

I found that this change (while improving reuse of request) causes regression with multi-slice streams when decoding on sliced based decoder (Cedrus/Allwinner). Reverting it from 1.18, I'll try and come up with a better solution for master. This notably fixes issues when using GStreamer 1.18 on Pine Phone.

This reverts commit a3e6d9fc