Prune reviewers from pkg/cloudprovider

Per discussion in https://github.com/kubernetes/kubernetes/pull/36530 the `OWNERS` file for `pkg/cloudprovider` should not contain additional reviewers at this time.

pkg/cloudprovider/OWNERS

@@ -1,43 +1,2 @@
 approvers:
 - mikedanese
-reviewers:
-- thockin
-- lavalamp
-- smarterclayton
-- wojtek-t
-- bgrant0607
-- deads2k
-- brendandburns
-- derekwaynecarr
-- caesarxuchao
-- vishh
-- mikedanese
-- liggitt
-- bprashanth
-- gmarek
-- erictune
-- davidopp
-- pmorie
-- sttts
-- quinton-hoole
-- dchen1107
-- saad-ali
-- zmerlynn
-- luxas
-- justinsb
-- roberthbailey
-- eparis
-- jlowdermilk
-- piosz
-- jsafrane
-- dims
-- krousey
-- rootfs
-- jszczepkowski
-- markturansky
-- vmarmol
-- girishkalele
-- satnam6502
-- jdef
-- freehan
-- jingxu97