Removes enable_mus from ozone-linux builder

Wayland support won't come from mus, so ozone-linux shouldn't turn on
mus.

BUG=none
TEST=none

Change-Id: I682f7998f702d9826f9266ab74266a2593e4d9fb
Reviewed-on: https://chromium-review.googlesource.com/1036192Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554941}

docs/ozone_overview.md

@@ -163,13 +163,13 @@ Then to run for example the headless platform:
 To build `chrome`, do this from the `src` directory:
 
 ``` shell
-gn args out/OzoneLinuxDesktop --args="use_ozone=true enable_mus=true"
+gn args out/OzoneLinuxDesktop --args="use_ozone=true"
 ninja -C out/OzoneLinuxDesktop chrome
 ```
 Then to run for example the X11 platform:
 
 ``` shell
-./out/OzoneLinuxDesktop/chrome --ozone-platform=x11 --enable-features=Mus
+./out/OzoneLinuxDesktop/chrome --ozone-platform=x11
 ```
 
 ### GN Configuration notes

tools/mb/mb_config.pyl

@@ -2008,7 +2008,6 @@
       'gn_args': ('ozone_auto_platforms=false ozone_platform_wayland=true '
                   'ozone_platform="x11" '
                   'ozone_platform_x11=true ozone_platform_gbm=true '
-                  'enable_mus=true '
                   'use_xkbcommon=true use_ozone=true'),
     },