doc: Remove references to GTK.

We don't support it.

src/nvim/ex_cmds.lua

@@ -1024,12 +1024,6 @@ return {
     addr_type=ADDR_LINES,
     func='ex_helpclose',
   },
-  {
-    command='helpfind',
-    flags=bit.bor(EXTRA, NOTRLCOM),
-    addr_type=ADDR_LINES,
-    func='ex_ni',
-  },
   {
     command='helpgrep',
     flags=bit.bor(EXTRA, NOTRLCOM, NEEDARG),

src/nvim/options.lua

@@ -1143,20 +1143,6 @@ return {
       varname='p_ic',
       defaults={if_true={vi=false}}
     },
-    {
-      full_name='imactivatefunc', abbreviation='imaf',
-      type='string', scope={'global'},
-      secure=true,
-      vi_def=true,
-      enable_if=false,
-    },
-    {
-      full_name='imactivatekey', abbreviation='imak',
-      type='string', scope={'global'},
-      vi_def=true,
-      enable_if=false,
-      defaults={if_true={vi=""}}
-    },
     {
       full_name='imcmdline', abbreviation='imc',
       type='bool', scope={'global'},
@@ -1193,13 +1179,6 @@ return {
         if_false={vi=macros('B_IMODE_NONE')},
       }
     },
-    {
-      full_name='imstatusfunc', abbreviation='imsf',
-      type='string', scope={'global'},
-      secure=true,
-      vi_def=true,
-      enable_if=false,
-    },
     {
       full_name='include', abbreviation='inc',
       type='string', scope={'global', 'buffer'},