highlight goBoolean and goPredefinedIdentifier groups using Identifier

Highlight the goBoolean and goPredefinedIdentifier groups as Identifer
instead of Boolean so that they will actually be highlighted, too.

The goBoolean and goPredefinedIdentifer groups were the only groups that
contained any of the predeclared identifiers
(https://golang.org/ref/spec#Predeclared_identifiers) that were not
highlighted out-of-the-box with vim-go.

If users want the previous (or different) behavior, they are free to
link the groups to different highlighting groups in their configs.

syntax/go.vim

@@ -53,8 +53,8 @@ syn keyword     goBoolean                  true false
 syn keyword     goPredefinedIdentifiers    nil iota
 
 hi def link     goBuiltins                 Identifier
-hi def link     goBoolean                  Boolean
-hi def link     goPredefinedIdentifiers    goBoolean
+hi def link     goBoolean                  Identifier
+hi def link     goPredefinedIdentifiers    Identifier
 
 " Comments; their contents
 syn keyword     goTodo              contained TODO FIXME XXX BUG