Revert unusable workaround for PAUSE being sent when going NULL

Directly setting rtspsrc to the NULL state before putting the pipeline
in the NULL state usually works, but it can cause a deadlock in some
cases, so it's not a reliable mechanism to fix this.

This reverts commit f37afdaf:
"rtspsrc: Fix state changes from PAUSED to PLAYING"

and commit 76d624b2:
"rtspsrc: Do not send PAUSE command when going to GST_STATE_NULL"

Part-of: <!908>