Remove custom dockerfile for bitpoll #16

Job	Run time
Build App Containers (limesurvey) / Build limesurvey	1m 31s
Build App Containers (karma) / Build karma	2m 2s
Build App Containers (bitpoll) / Build bitpoll	32s
Build App Containers (etherpad) / Build etherpad	15m 9s
Build App Containers (sogo) / Build sogo	3m 25s
Build App Containers (nextcloud) / Build nextcloud	1m 35s
	24m 14s

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