guru: drop guru support

Guru has been removed from golang.org/x/tools/cmd/guru as described in
https://docs.google.com/document/d/1_Y9xCEMj5S-7rv2ooHpZNH15JgRT5iM742gJkw5LtmQ/edit#heading=h.ojv16z1d1gas.

autoload/go/config.vim

@@ -141,28 +141,6 @@ function! go#config#InfoMode() abort
   return get(g:, 'go_info_mode', 'gopls')
 endfunction
 
-function! go#config#GuruScope() abort
-  let scope = get(g:, 'go_guru_scope', [])
-
-  if !empty(scope)
-    " strip trailing slashes for each path in scope. bug:
-    " https://github.com/golang/go/issues/14584
-    let scopes = go#util#StripTrailingSlash(scope)
-  endif
-
-  return scope
-endfunction
-
-function! go#config#SetGuruScope(scope) abort
-  if empty(a:scope)
-    if exists('g:go_guru_scope')
-      unlet g:go_guru_scope
-    endif
-  else
-    let g:go_guru_scope = a:scope
-  endif
-endfunction
-
 function! go#config#EchoCommandInfo() abort
   return get(g:, 'go_echo_command_info', 1)
 endfunction