Use ctags before cscope

cscope does not well support c++ (e.g. enum declared in extern "C"
https://sourceforge.net/p/cscope/bugs/249/, anonym struct ...)
So use ctags first (set csto=1)
also remove so duplicate code and simplify it as cscopetag option is
enabled

.vim/plugin/cscope.vim

@@ -5,27 +5,25 @@
 " if compiled with --enable-cscope
 if has("cscope")
 	if exists("g:loaded_cscope") || &cp
-    		finish
+		finish
 	endif
 	let g:loaded_cscope   = 1.0
 
 	function! GoToDefinition()
 		try
-			execute "cscope find g " . expand("<cword>")
-		catch /:E259:/
-			try
-				execute "tag " . expand("<cword>")
-			catch /:E257:/
-				execute "normal! gd"
-				execute "nohlsearch"
-			endtry
+			execute "tag " . expand("<cword>")
+		catch /:E257:/
+			execute "normal! gd"
+			execute "nohlsearch"
 		endtry
 	endfunction
+	nmap <silent> , :call GoToDefinition()<CR>
+	nmap <silent> ; <C-t>
 
 	" use both cscope and ctag for 'ctrl-]', ':ta', and 'vim -t'
 	set cscopetag
 	" use ctags before cscope
-	set csto=0
+	set csto=1
 	" add any cscope database in current directory
 	if filereadable("cscope.out")
 		cscope add cscope.out

.vimrc

@@ -219,21 +219,6 @@ set mat=5
 " go to declaration with F5
 " map <silent> <F5> gd:nohlsearch<CR>
 " imap <F5> <ESC><F5>i
-" try to go to definition or declaration with , and go back with ;
-function GoToDefinition()
-        try
-                execute "cscope find g " . expand("<cword>")
-        catch /:E259:/
-                try
-                        execute "tag " . expand("<cword>")
-                catch /:E257:/
-                        execute "normal! gd"
-                        execute "nohlsearch"
-                endtry
-        endtry
-endfunction
-nmap <silent> , :call GoToDefinition()<CR>
-nmap <silent> ; <C-t>
 
 " plugin taglist
 let Tlist_Ctags_Cmd = '/usr/bin/ctags'