main/postmarketos-ui-phosh: remove custom button-layout override (MR 2945)

Phosh takes care of it in a smarter way[1]. The custom override also has the
consequence of never showing closing buttons in docked mode, which can be
an inconvenience.

[1] https://gitlab.gnome.org/World/Phosh/phosh/-/blob/main/src/docked-manager.c#L292-L295

Or in case the lines change with time:
```C
phosh_docked_manager_set_enabled (PhoshDockedManager *self, gboolean enable)

  if (enable)
    g_settings_reset (self->wm_settings, WM_KEY_LAYOUT);
  else
    g_settings_set_string (self->wm_settings, WM_KEY_LAYOUT, "appmenu:");
```

main/postmarketos-ui-phosh/000-gschema.override

@@ -1,16 +1,12 @@
 [org.gnome.desktop.background]
 picture-uri='file:///usr/share/wallpapers/postmarketos.jpg'
 
-[org.gnome.desktop.wm.preferences]
-button-layout='appmenu:'
-
 [org.gnome.Epiphany]
 search-engines=[('Search the Web', 'https://duckduckgo.com/?q=%s&t=epiphany', ''), ('DuckDuckGo', 'https://duckduckgo.com/?q=%s&t=epiphany', '!ddg'), ('Qwant', 'https://www.qwant.com/?q=%s', '!q')]
 
 [org.gnome.Epiphany.web]
 user-agent='Mozilla/5.0 (Linux; Android 10; Pixel) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/83.0.4103.96 Mobile Safari/537.36'
 
-
 [org.gnome.settings-daemon.plugins.power]
 sleep-inactive-ac-type='nothing'
 ambient-enabled=false

main/postmarketos-ui-phosh/APKBUILD

@@ -109,7 +109,7 @@ firefox() {
 }
 
 sha512sums="
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