doc: drop PPC BE from supported platforms

We've discussed this with the Power team within IBM and have
agreement that we should drop Power Linux BE for Node version
8 as customers will be using Power Linux LE when using
Node.js going forward.

PR-URL: #12309
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: George Adams <george.adams@uk.ibm.com>
Reviewed-By: Jaideep Bajwa <bjaideep@ca.ibm.com>

BUILDING.md

@@ -40,7 +40,6 @@ Support is divided into three tiers:
 | Windows      | Tier 1       | >= Windows 7 or >= Windows2008R2 | x86, x64             |                  |
 | SmartOS      | Tier 2       | >= 15 < 16.4                     | x86, x64             | see note1        |
 | FreeBSD      | Tier 2       | >= 10                            | x64                  |                  |
-| GNU/Linux    | Tier 2       | kernel >= 4.2.0, glibc >= 2.19   | ppc64be              |                  |
 | GNU/Linux    | Tier 2       | kernel >= 3.13.0, glibc >= 2.19  | ppc64le              |                  |
 | AIX          | Tier 2       | >= 7.1 TL04                      | ppc64be              |                  |
 | GNU/Linux    | Tier 2       | kernel >= 3.10, glibc >= 2.17    | s390x                |                  |