ibm: rainier: remove witherpsoon, ac-server overrides

Rainier mistakenly has witherspoon and ibm-ac-server machine overrides
applied.  Rainier is neither a witherspoon nor an ac-server (accelerated
compute); remove so overrides approrpriate to those system types are not
applied during a Rainier build.

(From meta-ibm rev: c8e0f7ce18f89b09265ed4d2019f8a15999624a7)

Change-Id: I3e931d032832bff0191eca2f4431e4bff1de2406
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>