Remove outdated xgboost-cpu arch note. (#12169)

Author: trivialfis

doc/install.rst

@@ -94,8 +94,6 @@ You may choose to reduce the size of the installed package and save the disk spa
 The ``xgboost-cpu`` variant will have drastically smaller disk footprint, but does not provide some features, such as the GPU algorithms and
 federated learning.
 
-Currently, ``xgboost-cpu`` package is provided for x86_64 (amd64) Linux and Windows platforms.
-
 Conda
 *****