audiodec/enc: clear reconfigure flag if negotiate succeeds

So that it avoids to send an allocation query twice.
One from an early call to gst_audio_encoder_negotiate from a
subclass, then one from gst_audio_encoder_allocate_output_buffer.
Which means that previously gst_audio_encoder_negotiate was not
clearing the GST_PAD_FLAG_NEED_RECONFIGURE even on success.

Fixes bug https://bugzilla.gnome.org/show_bug.cgi?id=719684
videodec/enc: clear reconfigure flag if negotiate succeeds

So that it avoids to send an allocation query twice.
One from an early call to gst_video_encoder_negotiate from a
subclass, then one from gst_video_encoder_allocate_output_frame.
Which means that previously gst_video_encoder_negotiate was not
clearing the GST_PAD_FLAG_NEED_RECONFIGURE even on success.

Fixes bug https://bugzilla.gnome.org/show_bug.cgi?id=719684