Remove taint support

Ruby 2.7 deprecates taint and it no longer has an effect.
The lack of taint support should not cause a problem in
previous Ruby versions.

Author: jeremyevans

ext/stringio/stringio.c

@@ -623,7 +623,6 @@ strio_copy(VALUE copy, VALUE orig)
 	strio_free(DATA_PTR(copy));
     }
     DATA_PTR(copy) = ptr;
-    OBJ_INFECT(copy, orig);
     RBASIC(copy)->flags &= ~STRIO_READWRITE;
     RBASIC(copy)->flags |= RBASIC(orig)->flags & STRIO_READWRITE;
     ++ptr->count;
@@ -1436,7 +1435,6 @@ strio_write(VALUE self, VALUE str)
     if (ptr->pos == olen) {
 	if (enc == ascii8bit || enc2 == ascii8bit) {
 	    rb_enc_str_buf_cat(ptr->string, RSTRING_PTR(str), len, enc);
-	    OBJ_INFECT(ptr->string, str);
 	}
 	else {
 	    rb_str_buf_append(ptr->string, str);
@@ -1445,9 +1443,7 @@ strio_write(VALUE self, VALUE str)
     else {
 	strio_extend(ptr, ptr->pos, len);
 	memmove(RSTRING_PTR(ptr->string)+ptr->pos, RSTRING_PTR(str), len);
-	OBJ_INFECT(ptr->string, str);
     }
-    OBJ_INFECT(ptr->string, self);
     RB_GC_GUARD(str);
     ptr->pos += len;
     return len;

test/stringio/test_stringio.rb

@@ -180,17 +180,6 @@ def o.to_s; "baz"; end
     f.close unless f.closed?
   end
 
-  def test_write_infection
-    bug9769 = '[ruby-dev:48118] [Bug #9769]'
-    s = "".untaint
-    f = StringIO.new(s, "w")
-    f.print("bar".taint)
-    f.close
-    assert_predicate(s, :tainted?, bug9769)
-  ensure
-    f.close unless f.closed?
-  end
-
   def test_write_encoding
     s = "".force_encoding(Encoding::UTF_8)
     f = StringIO.new(s)