Removed function used with ruby3.2.0preview3 for ruby-shadow 1.5.1

[mtasaka]

Even after #31 is merged, make shows some implicit functions are used:

gcc -I. -I/usr/include -I/usr/include/ruby/backward -I/usr/include -Ishadow -DHAVE_GETSPENT -DHAVE_FGETSPENT -DHAVE_SETSPENT -DHAVE_ENDSPENT -DHAVE_LCKPWDF -DHAVE_ULCKPWDF -DHAVE_SGETSPENT -DHAVE_RUBY_IO_H    -fPIC -DRUBY19 -m64 -o shadow.o -c shadow/shadow.c
shadow/shadow.c: In function 'convert_pw_struct':
shadow/shadow.c:37:23: warning: implicit declaration of function 'rb_tainted_str_new2'; did you mean 'rb_enc_str_new'? [-Wimplicit-function-declaration]
   37 |                       rb_tainted_str_new2(entry->sp_namp),
      |                       ^~~~~~~~~~~~~~~~~~~
      |                       rb_enc_str_new

So make seems to finish without "error", however generated shadow.so is actually broken.

This is due to https://github.com/ruby/ruby/commit/39bc5de83394a380c9967df74e06c824186f8560 , taintedness was already announced to be removed with ruby3.2.

Note that ruby3.2 is expected to be released on 2022-Dec-25.