doc: remove SmartOS from official binaries

Based on feedback from Joyent, the SmartOS builds are largely unusable
without bundling all of the libraries used to build the binary so are
being dropped starting from Node.js 14. We will still test SmartOS in
the CI.

Signed-off-by: Richard Lau <riclau@uk.ibm.com>

PR-URL: #32812
Refs: nodejs/build#2168
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: richardlau

BUILDING.md

@@ -175,7 +175,6 @@ Binaries at <https://nodejs.org/download/release/> are produced on:
 | linux-ppc64le         | CentOS 7 with devtoolset-6 / GCC 6 <sup>[7](#fn7)</sup>                  |
 | linux-s390x           | RHEL 7 with devtoolset-6 / GCC 6 <sup>[7](#fn7)</sup>                    |
 | linux-x64             | CentOS 7 with devtoolset-6 / GCC 6 <sup>[7](#fn7)</sup>                  |
-| sunos-x64             | SmartOS 18 with GCC 7                                                    |
 | win-x64 and win-x86   | Windows 2012 R2 (x64) with Visual Studio 2019                            |
 
 <em id="fn7">7</em>: The Enterprise Linux devtoolset-6 allows us to compile