Remove existing continuous benchmark infrastructure (#6239)

It hasn't worked for quite a while. We can come back to this in the future, but it is probably best to completely remove this for now, and then use this PR as a reference if we come back to it.

Author: DavisVaughan

.Rbuildignore

@@ -5,7 +5,6 @@
 ^.Rprofile$
 ^inst/db$
 ^inst/bench$
-^bench$
 ^man-roxygen$
 ^demo/pandas$
 ^\.httr-oauth$