Delete the default Token assignment opoerator

Recent Clang issues a warning for this:

  warning: definition of implicit copy assignment operator for 'Token' is deprecated because it has a user-declared copy constructor

Change-Id: I61345d30ff50fd51c587d1a6fbebd777ae3cbd60
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/6760
Commit-Queue: Petr Hosek <phosek@google.com>
Reviewed-by: Brett Wilson <brettw@chromium.org>

src/gn/token.cc

diff --git a/src/gn/token.cc b/src/gn/token.cc
index c648371..0466ce7 100644
--- a/src/gn/token.cc
+++ b/src/gn/token.cc
@@ -11,8 +11,6 @@
 Token::Token(const Location& location, Type t, const std::string_view& v)
     : type_(t), value_(v), location_(location) {}
 
-Token::Token(const Token& other) = default;
-
 bool Token::IsIdentifierEqualTo(const char* v) const {
   return type_ == IDENTIFIER && value_ == v;
 }

src/gn/token.h

diff --git a/src/gn/token.h b/src/gn/token.h
index 8837466..c074620 100644
--- a/src/gn/token.h
+++ b/src/gn/token.h
@@ -59,7 +59,6 @@
 
   Token();
   Token(const Location& location, Type t, const std::string_view& v);
-  Token(const Token& other);
 
   Type type() const { return type_; }
   const std::string_view& value() const { return value_; }