Remove mention of $SAFE and taint from doc [ci skip]

doc/extension.ja.rdoc

@@ -215,17 +215,6 @@ rb_str_new_literal(const char *ptr) ::
 
   Cのリテラル文字列からRubyの文字列を生成する．
 
-rb_tainted_str_new(const char *ptr, long len) ::
-
-  汚染マークが付加された新しいRubyの文字列を生成する．外部
-  からのデータに基づく文字列には汚染マークが付加されるべき
-  である．
-
-rb_tainted_str_new2(const char *ptr) ::
-rb_tainted_str_new_cstr(const char *ptr) ::
-
-  Cの文字列から汚染マークが付加されたRubyの文字列を生成する．
-
 rb_str_append(VALUE str1, VALUE str2) ::
 
   Rubyの文字列str1にRubyの文字列str2を追加する．
@@ -1251,7 +1240,6 @@ Data_Get_Struct(data, type, sval) ::
   RB_INTEGER_TYPE_P(value)
   RB_FLOAT_TYPE_P(value)
   void Check_Type(VALUE value, int type)
-  SafeStringValue(value)
 
 === 型変換
 

doc/extension.rdoc

@@ -190,16 +190,6 @@ rb_str_new_literal(const char *ptr) ::
 
   Creates a new Ruby string from a C string literal.
 
-rb_tainted_str_new(const char *ptr, long len) ::
-
-  Creates a new tainted Ruby string.  Strings from external data
-  sources should be tainted.
-
-rb_tainted_str_new2(const char *ptr) ::
-rb_tainted_str_new_cstr(const char *ptr) ::
-
-  Creates a new tainted Ruby string from a C string.
-
 rb_sprintf(const char *format, ...) ::
 rb_vsprintf(const char *format, va_list ap) ::
 
@@ -1210,10 +1200,6 @@ void Check_Type(VALUE value, int type) ::
 
   Ensures +value+ is of the given internal +type+ or raises a TypeError
 
-SafeStringValue(value) ::
-
-  Checks that +value+ is a String and is not tainted
-
 === Data Type Conversion
 
 FIX2INT(value), INT2FIX(i) ::

doc/security.rdoc

@@ -15,19 +15,6 @@ mailto:security@ruby-lang.org ({the PGP public
 key}[https://www.ruby-lang.org/security.asc]), which is a private mailing list.
 Reported problems will be published after fixes.
 
-== <code>$SAFE</code>
-
-Ruby provides a mechanism to restrict what operations can be performed by Ruby
-code in the form of the <code>$SAFE</code> variable.
-
-However, <code>$SAFE</code> does not provide a secure environment for executing
-untrusted code.
-
-If you need to execute untrusted code, you should use an operating system level
-sandboxing mechanism. On Linux, ptrace or LXC can be used to sandbox
-potentially malicious code. Other similar mechanisms exist on every major
-operating system.
-
 == +Marshal.load+
 
 Ruby's +Marshal+ module provides methods for serializing and deserializing Ruby